What would you do? relationship question?!?

A couple months ago my husband was giving our 1 year old a bath and his phoe was sitting on the table next to me and he got a text message. I looked at it and it was from a girl that he works with (I only knew this because her name in his phone said Janelle-work which was weird because why would he have her in ther like that?) It was a name and number I never saw before and the text said :" Hey you! What are you up to tonight?" Now I know I shouldnt have done this, but I texted her back pretending to be him and said " Nothing just home with my wife and son" Now she then texted him back things basically saying that she wanted him to come to her house and she ured him she "has no ties just an ex boyfriend" So basically at this point I was fuming and confronted my husband and it turned into this whole big argument. He swears he was only talking to her about work and had no idea why she would be texting him things like this. I texted to her from my phone then and told her it was me the whole time and what the heck is going? She told me he had given her his numner and he was the one pursuing her. She told me nothing had happened yet and she was sorry and she wouldnt talk to him anymore. I honestly believe her and he will never admit to me the truth. So basically its a couple months later and I still think about this everyday and I just want the truth and don't know what to do. Any suggestions? Should I just let it go? Or should I leave him. I get mad alot about this and we fight and acts like I am just being a b*tch for no reason.