Do I end it with him?

Early in our relationship I had asked him about a girl name Christine who always snapchatted him. He said it was nothing to worry. So I didn't. Until I saw their txt and figured out she was a tinder match that he lied to me about and whom would occasionally hit him up. We had a big fight, he blocked her on snap and phone number. Last night she requested to follow him on Instagram and he accepted and followed back. I got really upset because clearly I don't want this person around as a threat and it seems like a careless act. He has blocked her there to and apologized all night. Am I overreacting or is this BS like I think it is?