I feel girlfriend is too friendly with my cousins..?

She has told me before she was into the thuggish gangster kinda style and she has even asked me if how come I don't dress like that. But last nite I hung out with her and my cousin and his girlfriend and my cousin is more gangster style, and I was noticing that whatever he would say she would laugh at it but when I say something she would just kinda smile. At one point, we were racing with our girlfriends with piggy back rides and I fell with her on top of me and she even started laughing which I felt pretty bad about instead of seeing if I was alrite first. During the nite, my cousin also said something like, "I wish I would have brought my weed" and she was all excited saying YEA! When I've clearly told her I don't want her smoking weed anymore. All this stuff bothered me and felt like she really changed alot due to hanging out with my cousin. What should I do?