Stomach Aches, Vomitting,Headaches etc. Not pregnant could the doctor be wrong?

I have stomach aches very often. Most of the time it is tolerable pain. However, sometimes it is outragiously painfull. My are alot more tender than normal. I also have back pain.Every day i wake up and want to vomit. Also im very sensitive with eating. Which is unusual, Meaning after every meal i want to vomit. I cant swallow Advil because it also makes me puke. I have headeaches occasionally. Also when i get up to quickly i get dizzy and light headed. I am a little bit more moody then normal. My period only lasted two days this time, and i only bleed after i had .. For two days. After that i havent bleed since. I went to the doctor they just checked to see if i was pregnant and it was negative is there a chance i am? Or whats going on with me? Also im 15 yrs old. Have been having for a while. And no its not cramping. My actual stomach is aching not my lower stomach. Its not like any Period cramp i have ever felt. So its not that... Please Help.. Thanks in advance.