I need advise. girlfriend issues.?

okay we have a complicated relationship. weve been talking for like 4 months but only been dating for like one. i asked her out before then but she said now. we are together now and she claims to be in love with me. we used to have sex like all the time. weve have sex like once since weve comitted to a relationship. i asked her why and she says shes self concious becuase shes gaining wieght. DONT BELIEVE IT. she barely kisses me. we went to a party and she started rollin around on the floor makin out with this chick and shakin her *** all on some dude knowing i was pissed off. i left her a comment on her facebook yesterday saying happy easter baby i love you. she didnt reply to it even tho she was replying to picture comments other people were leaving her. what should i do??? please help