Hello BethEllen and welcome...sorry to hear you are suffering like this! Your story sounds very similar to mine. This might be long-winded, but I hope it gives you a little hope and maybe some suggestions...
When I lived in Africa five years ago, I came down with violent diarrhea, vomiting, and high fevers every two weeks for MONTHS. It was absolute hell on earth! When I got back to the U.S. and had a colonoscopy and EGD done, they said everything looked clear. I had megadoses of IV Flagyl/Cipro and anti-malarials for several days and did get better...for a while.
But. I proceeded to get sick with diarrhea and fevers for the next five years on an intermittent basis. It was tolerable enough to pass off as the stomach flu, generally, as it would only last a week or so each time. But two years ago I began dropping weight at an alarming rate (went from 175 to 145 in something like three months), was having diarrhea 30x a day - consistently - with blood and leukocytes in my stools, fevers up to 103, and crazy abdominal pain. I missed so much work I had to go on medical leave. I went to my primary care doc, who ordered me to see a GI, who ordered me to see an infectious disease specialist, who ordered me to go to the Mayo Clinic to see another GI and ID specialist. It was insane (just ask the ladies on this board; I "met" them around that time!).
I got NO answer whatsoever, not even from Mayo, where I continued being sick and they saw it firsthand. They were flabbergasted. They couldn't find a parasite. They couldn't find any evidence of inflammatory bowel disease from my scopes. Nada! I left Minnesota in tears! I came back home and honestly lived on Tylenol and Imodium for approximately another year. I tried to mask the symptoms as best I could.
I started seeing a nurse practitioner about a year ago, and she said, "Why haven't they just treated you for parasites, even if they can't see them?" So, she put me on a 10-day regimen of Flagyl and Cipro (it was a rough 10 days; that combo can make you feel pretty ill). My symptoms cleared up for something like 2 months. It was AMAZING. Two months later, the symptoms came back. She put me on Flagyl and Cipro again, another 10-day course.
That last course was 3-4 months ago. And I swear I have never felt better since the day I left for Africa more than five years ago.
I'm going to medical school next year and have some experience in parasitology, some of which I studied in college - parasites are notoriously difficult to track down. They ask you for a stool sample to find them, but they don't really understand (especially in the U.S.) that you often need more than three samples to have even a remotely good chance of finding the parasite, which only sheds its eggs intermittently in stools.
I would suggest, if you don't get another answer, go back to your doctor and ask about a 10-day Flagyl/Cipro combo. This is ESPECIALLY if you have done any international travel within the past couple of years.