what could be wrong with my stomach? HELP! PLEASE?!

okay so my stomach feels really weird, it mainly started when i got my period. i have a weird sensation in it all the time not and also on the side of my uterus area as well when i sit down. i'm getting a burning feeling in my throat and abdomen and like a tight sort of feeling and kinda like butterflies in my lower part of my stomach. Also my lower right side of my back hurts somewhat. Another thing is i don't really feel hungry anymore, and i've lost some weight. I went to the doctor a little while ago for a physical and they said everything was okay. But i don't know what it might be, any ideas?