Did he cheat? And why?

We've been married for 2 years now and everything seemed to go well, we have a beautiful baby girl together. My husband has problems with lying, he always lies, but it's always something minor and every time I just seem to get over it... But two days ago I found a message in his cell phone asking some woman to come by his work for a "quicky"... i asked him what thats all about and at first he didn't said anything, and than he said it was a joke cause this woman is always flirting with him and he was just teasing her but nothing happened. I might would believed him but after knowing how easy is for him to lie I don't think I do believe him. I really want to but I have this gut feeling that he is lying and have cheated. I don't understand why would he do it though. We have great sex life, he doesn't have to go out ask for it, i've even told him recently that I would fulfill his fantasy of having threesome cause I know thats something he really wants. We don't have any other problems in marriage, no financial problems, we rarely, almost never fight... So, do you think he cheated? If so, why would he do that? Sorry for long post and tnx for answers in advance.