Relationship problems

Hi there.
I've been dating my grfiend for 4,5years, and it's been great. we've had our ups and downs, but great.
she's a really nice girl with 23years. she had one previous relashionship, +5years. she cheated on him twice: once when she was really young and the other with me. she didnt tell her ex that she cheated on me and told me that that are truths that are better if u dont tell it.

so. we've been living together for 3years, with the same roomate. He is a nice guy, and he has become a close friends of me and my gfriend.
One night, they had a faculty party ( im not in the same faculty) and we met really late that night. we were all drunk and stuff.
at one point, i was dancing with my gdriend and a girl went to talk with our roommate and she went rerally quick to them and i got the sensation that she was pushing the other girl away. i fekt an horrible sensation.
i talked about it with her some days later and she said she didnt remember the moment, it was in my head, ect.
the situation now is:
our rommate was 1month out and returned some days ago.
i can see that she is constalty looking at him while we are on the living room, on the street playing games like pushing each other (touching each other). all her body expressions are centered on him. she laughs in a silly way at jokes that he says that arent that funny or is the 5th time he says it.

is she falling for him?