Sick off practically all foods!

Since I was around 15 I have been feeling/being sick regularly and having severe head and eye pain, roughly every couple of days. I have been to the doctors lots of times about it and so far had 2 urine tests, 3 blood tests and an ultrasound and there's been nothing wrong, I've also been given glasses but turned out my eye sight was perfect. I got told to avoid acidy food and citris acid by my doctors too which is impossible when everything else makes me sick. Every other drink makes me ill besides coke and pepsi (which makes no sense to me either, even water...) and I can only eat fish and chicken in meats, pasta and vegetables. I keep trying to explain that I can eat hardly anything but he doesn't get it at all. Most recently I've been put on migrane tablets which cut down how often I am sick, but it is still every 10 days. Any ideas to what it would be