The American-English expression Pharaoh’s Revenge (also Pharaoh’s revenge and pharaoh’s revenge) denotes diarrhoea suffered by travellers, especially in Egypt.
—British-English synonym: gyppy tummy.

Two remarks:
1-: Nothing in the texts containing the early occurrences of Pharaoh’s Revenge suggests that this expression was coined after Montezuma’s Revenge, which is first recorded in 1955 and denotes diarrhoea suffered by travellers, especially in Mexico.
2-: There is perhaps, in the expression Pharaoh’s Revenge, an allusion to the legendary curse of the pharaohs, i.e., a curse alleged to be cast upon anyone who disturbs the tomb of a pharaoh.

The expression Pharaoh’s Revenge occurs, for example, in Josh Freed on Obamamania, by Josh Freed, about U.S. President Barack Obama’s visit to Canada, published in The Gazette (Montreal, Quebec, Canada) of Saturday 21^st February 2009:

WE’RE SAFE: We are the perfect foreign country to test out Obama’s Secret Service, a gun-shy land where no one throws snowballs, let alone shoes. Our food is safe, too. Unlike visiting Mexico, there’s no chance that an errant presidential nibble will give him Montezuma’s Revenge, or Gringo Gallops, not to mention more exotic stomach rumbles in other countries—like Delhi Belly, Pharaoh’s Revenge or Thai-dal Wave.

The earliest occurrences of the expression Pharaoh’s Revenge and variants that I have found are as follows, in chronological order:

1-: From Nile Steamer Tours Easy and Informal, about cruises on the Nile in Upper Egypt, by Marcus Brooke, published in the Los Angeles Times (Los Angeles, California, USA) of Sunday 8^th April 1973:

A few hints may add to the pleasure of your cruise.
—Chances are you will be smitten—although not on the steamer—with pharaoh’s revenge so be prepared with suitable medication.

2-: From Kissinger Notebook: Tight Security Marks Middle East Travels, the account of a mission to the Middle East undertaken by the German-born U.S. diplomat Henry Kissinger (born 1923), then Secretary of State—account by Wilbur G. Landrey, UPI Foreign Editor, writing from Jerusalem, published in The Shreveport Times (Shreveport, Louisiana, USA) of Sunday 12^th May 1974:

Some things go wrong—one reporter is left behind on the beach in Cyprus when the plane leaves early. Another is attacked by the “Pharaoh’s Revenge” on the press bus in Egypt and had to stop twice to go behind the sand dunes.

3-: From a portrait, by Joy Billington, Star-News Staff Writer, of Nancy Kissinger (née Maginnes – born 1934), Henry Kissinger’s wife, published in the Washington Star-News (Washington, District of Columbia, USA) of Friday 7^th June 1974:

She talks about the Middle Eastern trip that was her initiation as the wife of the Secretary of State.
“I had, by far, the best time. I didn’t have to work, as Henry and the accompanying press did. I had all the top archeologists in every country, and I could go where I pleased.”
A one-day bout of “Pharaoh’s Revenge” was well worth the “incredible sight” of Abu Simbel rising on the banks of the Nile.

4-: From the Los Angeles Times (Los Angeles, California, USA) of Friday 27^th December 1974:

Pharaoh’s Revenge: Harvard Loses on Nile
BY JACK FOISIE
Times Staff Writer

CAIRO—A combination of Pharaoh’s revenge and eight stalwart Egyptian policemen ended Harvard’s supremacy on the Nile River Thursday.
In the annual rowing competition on the famous waterway where Cleopatra’s barge was propelled by slaves, the eight-oared crews of Harvard, Yale, Cambridge and Oxford were defeated by rowers from the Cairo police force.
[…]
Harvard’s coach, Harry Parker, made no excuses, although the team’s stroke, Olympian Al Sheely, was sidelined with a heavy cold and several other crewmen had been weakened by upset stomachs, caustically referred to by foreigners here as the Pharaoh’s revenge.

5-: From Step back in time and cruise the Nile, by Margaret Piton, of The Gazette, published in The Gazette (Montreal, Quebec, Canada) of Saturday 18^th January 1975:

CAIRO—Travellers whose idea of a holiday is to visit a place very different from their homes might consider a cruise along the Nile in Upper Egypt.
[…]
Travellers are advised to take the usual precautions with regard to food and drinking water, but some form of “Pharaoh’s revenge” is considered almost inevitable.

6-: From Perfect Pigeon: An epicure’s delight, by Marq de Villiers, published in several Canadian newspapers on Saturday 19^th April 1975—for example in The Calgary Herald (Calgary, Alberta, Canada):

We only found those delectable birds by wandering off Cairo’s thoroughly beaten tourist path, after resolutely ignoring the warnings of a shrill-voiced South African, who warned us never, never, never to touch local food. Muttering something about idiot prejudices, we set off, and duly found a quaint restaurant where they’d never heard of Yorkshire pudding (or so we assumed, as they couldn’t speak English and we couldn’t speak Arabic) and we congratulated ourselves on how authentic it all was. It was, too: for the next four days we smiled gamely and chattered about how marvelous the food had been, and between brave chuckles dashed downstairs, where Pharaoh’s Revenge could be suffered in decent obscurity—we weren’t going to give that South African her satisfaction, though Pharaoh sure got his.

7-: In his column The Sporting Thing, published in The Seattle Times (Seattle, Washington, USA) of Wednesday 18^th June 1975, Georg N. Meyers, Sports Editor, evoked Harvard’s defeat in the above-mentioned rowing competition on the Nile (cf. quotation 4):

It may have escaped notice that, last December, at the invitation of Egypt, Parker took his oarsmen to Cairo to race a boatload of muscular specimens identified—honestly—as policemen.
On the fabled Nile, where Cleopatra barged, the Cairo cops beat the crimsonned Crimson, Harvard’s only defeat in two years. The losers felt they had a legitimate excuse.
At least six of the visiting oarsmen, after two days of sightseeing, were suffering from a malady known as Pharaoh’s Revenge—or, with the characteristic Harvard decorum, the Pyramid Poops.