Is he cheating on me with her?

my boyfriend and i have been dating for a little over 2 months now. 2 days after he asked me out he came to school with hickies all over his neck that i didnt give him. he claims that he got them the night before he asked me out.this might be true because he asked my saturday and i didnt see him till monday. a week after that, i was told by several people that he and my ex best friend made out at the park. they had dated for a few days a couple years back and had been friends with benifets after that. when i asked him about it he told me they nevr made out, but he did smoke weed with her and gave her a mouth to mouth p. i didnt get to upset by that and believed him. since then, everytime the 3 of us hung out she would non stop flirt with him and look at me while she did it. which pissed me off. another time we hung out he drew a heart on her leg with me sitting there. i recently came up to illinois to see family while he is still in texas..with her. one day she messaged me on facebook to ask me how i was so i thought i would try to rub it in and said 'great, since dustin and i have been so close lately.' and then she said 'yall r still dating?' and i said 'of course, its almost been 2 months.' thats when she told me that he told her that he and i were no longer dating and that he loved her, not me.& how sorry he was for being so rude to her.i of course freaked out and started bawling my eyes out.later in the week i finally called him and asked him about this..this was his exscuse : "i asked her out as a joke. i made a bet with her ex boyfriend that i could get her to go out with me. i never told her i loved her and i never said anything about you. i did tell her we werent dating so that she would go out with me." i asked him why he didnt just tell me about the joke and he said that i would have freaked out and gotten mad. i believed his story. but today i heard from my friend that she always sees them hanging out.. in the woods!! i also saw on facebook that she posted on his wall 'i love you'. i dont wanna break up with him because i lost my virginity to him and i love him with all my heart..but i dont wanna stress about it anymore. he swears to me he has never cheated on me. should i believe him? or is he cheating on me with her?