I have a problem!!!!?

ok about a month ago i went out of town and my boyfriend said he went to movies fri night and sat night he told me both times he went with his friend but i found out he went with a nother gurl he use to talk to b4 i was with him... last night we went to the movies with are sis and i saw in his phone "gurlfriend" and it was the same gurl we been going out for 3 1/2 mon and idk what to do!!!