Do you think he Cheated on me? please help?

Ok so here's the story, My boyfriend and I are together just over a year, we also work together, he is a supervisor and I am a sales assistant, around May this year he had to transfer to another store temporarily to fill in for a manager who was sick, and in that time he told me that a female supervisor was flirting with him and trying to get him to go out with her, now he told me that himself straight away when it started happening so that I knew the truth but he says he didn't do anything with her he said he told her to stay well away, but then a girl from that store came to ours and started telling people that they had gone out and of course it got back to me, and I saw very little of him for the month he was there, we normally see each other every other day but he said he was too busy with work, when I asked him about what this girl was telling people he swore that it was all lies and I chose to believe him and not let rumors get in the way and destroy our trust in one another, now he's back in our store now since June but just today he came up to me in work and said that the girl who was flirting with him has started a rumor that he (my boyfriend) and her have slept together, he found out about the rumor from our store manager who is married to this girls store manager, she told him about it so he told my boyfriend the rumor was being made by this girl,

my boyfriend came straight to me to tell me in case it got back to me and he says nothing happened, but if nothing did happen then why would she start saying she slept with him after all these months? nothings been the same with me and my boyfriend since he went to the other store all we do is fight since he came back, should I believe him? I just don't know why she would say it all again after all this time? What should I do? Should I believe he is being truthful?