I've had the exact same thing since my early 20's (I'm 29 now). I went through several tests and the doctors could not find anything. Every time I'd diet I would lose 5 lbs or so then get super dizzy, queasy, shaky etc. Eating would make me feel better. At one point it was so bad I could not walk and was throwing up until my boyfriend at the time took me to the emergency room. The doctor there said his wife who was a nurse had similar problems and that she finally self-diagnosed herself with hypoglycemia. I said I'd been tested for it and it was negative and he told me it's very very hard to test for. He said to eat less sugary or refined foods (I was actually eating pretty healthy but was drinking a lot, since these were my university days
So I read up a lot about it and realized my symptoms did fit those of hypoglycemia. I was also a vegetarian at the time so I started eating meat to up my protein and that helped along with cutting down on the alcohol. Although it helped it wasn't cured and it took me going to a naturopathic doctor who then told me I had candida overgrowth. I'm not sure if I buy this one (seems like a fad thing) but I do strongly believe that my high carb (from being vegetarian) and alcohol diet HAD messed up my intestinal flora. So I followed the diet and that helped (you can google candida diet and find loads of info on it). I have completely cut out all grains except occasionally quinoa, and don't eat any dairy or sugar or refined foods. I eat a lot more healthy fat now too and take omega 3 supplements. I also take acidophilus to help replace the good bacteria. Now, for the first time in years, I can lose weight without feeling like absolute poop!
Anyways, sorry for the long-winded answer but your post reminded me of exactly what I used to feel like. Hopefully something I said will help you.