My boyfriend might be cheating on me?!? PLEASE HELP!10 POINTS FOR BEST ANSWER!?

okay so today is tuesday right.. well my boyfriend broke up with me sunday and then begged for me back because he said that he was going through alot because it was the weekend his grandma died and he was really close to her..so i forgave him for that and we were going back out again. but the day before he broke up with me he went to his friends birthday party and there was drinking and other girls there.. so i let him go to that and thenn today (tuesday) this girl walked up to me in 2nd period and goes "i really have to talk to you about something.." and i was like "what" and she goes "you know how your boyfriend went to that party yesterday and i was like "yyeeaaah?" and she was like "okay well we were all drunk and I started talking bad about you and so were some of my friends.. because I didnt like you then but now we are cool.. but anyways i was talking bad about you and everything i said, your boyfriend was agreeing with me and laughing and then we were sitting on the couch and he kept moving closer to me and he put his hand on my thigh and i was like uuhhh no get the **** off of me, you have a girlfriend and he said yeah well im gonna break up with her anyways.. and you dont like her so why does it matter? and then she supposably said no and walked away" and then i blew up on my boyfriend and broke up with him and stuff and he is denying touching her and is denying saying that he is gonna break up with me but he said he was just agreeing with them to get them to shut the fuc* up... i dont know what to believe.. please help me because i really loved this guy and i need to know the truth..and now hes texting me saying ill talk to you after school about this.. should i just move on or try and find out the truth and give him a second chance? please please please help me.. im desperate for some answers.