Hi Lisa
I sometimes get the same feeling as you and I can only eat small "meals" now. I have had test after test done and seen so many people in different fields, it's a joke!
I have been told that I have IBS but I'm not convinced, I think that if Dr's can't find out what's wrong they give up and just fobb you off with "Oh it's IBS" 
I've had CT scans (one with contrast dye), MRI scan, LOADS of bloods taken, 2 Endoscopies, Barium follow-though, lung function test, Synacthen Test, Alpha-1 test.
and NONE of them came back as abnormal.
My last resort was to see a Professor of Neurogastroenterology up in London hospital. He said it could be an overgrowth of bad bacteria in the gut. I am now on a 7 day course of Amoxicillin (you can see my reent post for my experience on that so far lol!)
In the meantime I've had dizziness, headaches, migraines, LOADS of nausea, diarrheoa, irritability, weight loss, lack of appetite etc etc.
See if you can get more tests done to rule out anything sinister like Crohns or Celiac Disease.
I Wish you luck X