Is he CHEATING on me??

A while back I found a pair of panties in my bedroom and they weren't mine. My boyfriend and I were living with my sister at the time so I thought they were hers. He claims he's never seen them before and when I asked my sister about them....she had no idea. They weren't hers either. Then about 4 months ago he decided to go play basketball with his buddy Skyler. The next day I found a headband in our truck. I don't wear headbands and when I asked him about I got the same response....."Uhhh...Ummm....I've never seen it before." Then he said that Skyler picked it up and put it in the truck. 2 weeks ago he went to a luau party with his grandparents. Come to find out....the party was for a REALLY PRETTY Girl that was turning 16. When he came home he had perfume on his neck. I asked him......and the same response.....then he said that a girl(which I know) named Nicole(happily married) sprayed it on him and said "oh here....let's make Jess mad." WTF????? HELP!!!!!! What should I do?