Idk what to think 😔

So there is this chick that my boyfriend used to sleep with before him and I got together. Well he stopped talking to her and everything then out of the blue just shows up at his house with another chick. He said that he smoked with them and then they left. This was within two hours of not hearing from him. I asked him why he didn’t tell me until they left and he said because I would of over reacted and went to his house. FYI she doesn’t know he has a girlfriend. Am I wrong for being mad and thinking he hooked up with her?