recurrent nausea after traveling to India

While I was in India, I became very ill - fever, stomach pain, nausea (very little actual vomiting), and lightheadedness.  This lasted at least a week, and I was a long time recovering afterward.  I was never diagnosed with anything or treated.

Now, a year later, I'm having recurrent bouts of nausea that are really affecting my work.  I have no fever and very rare vomiting; just epigastric discomfort - like a heavy rock in my stomach - and nausea.  This lasts about 1-2 weeks and then goes away.  I went to several doctors, had an upper GI scope, which showed nothing.  I'm on a PPI which doesn't seem to be helping, nor do antacids.  It doesn't seem to be brought on by any special foods or stressors.  I know when I got back from India, I had some blood tests done, but I don't remember what they were - just that they were all negative.

Any suggestions on what my next step should be?  I'm otherwise a healthy, 25-y/o female whose job is being ruined by my being persistently too ill to work efficiently.