Credit: U.S. Navy Photographer

For Jessica Simpson, even a simple visit with troops aboard the USS Harry S. Truman was far from smooth sailing.

"She flew in on a helicopter and had three Red Bulls with no water," a source tells the new Us Weekly of her Oct. 1 visit to the Persian Gulf.

"Then she complained it was too hot out." (It was 97 degrees that day in nearby Doha, Qatar.)

From there, things got messy. After eating, the singer, 30, attempted to perform. "She only sang one song, then vomited," says the insider. See more pics of Jessica on the high seas.

Still, a source close to Simpson adds, "She was dehydrated and got sick before she performed -- and even got an IV. The doctor told her not to sing, but she did anyway."

Back in the States on Tuesday, Simpson Tweeted her thoughts on the trip: "Feel so humbled by the sacrifice the troops and their families make to keep us safe. I'm forever changed after this last week. Thank you!"