My best friend who is a guy has a girlfriend and she thinks he likes me!?

So the guys name is Terry and him and I are amazing friends, we share every thing together he's like a guy and a girl smoshed in to one :) but he has a girlfriend named Cheyenne and she has bin dating Terry for about 1 week and she thinks Terry likes me. He use to like me ( like have a crush ) and she believes he still likes me, me may or may not but I also have a boyfriend ( of 4 and a half months ) and Cheyenne doesn't believe me that I dont like Terry, so how do I get on her " good side " with out breaking me and Terrys friendship?! HELP!