Is my boyfriend cheating on me?

Okay soo when ever im with him he just stop by and says hi then he makes up some lame excuse and says like he has to go to the doctors or something and all hes wanted us to go is have sex and stuff so i do but he leaves right after and on his birthday i went to give him a happy birthday surprise there was a girl sucking his d!ck he said that he will never do it again but idk please dont leave any rude ansers