Should I be mad?

So I got in a very serious fight with my boyfriend and we had broken up. The same night he went and had sex with someone else and then the next day he had called me up and fixed things with each other.

He didn't tell me he had sex with another girl until I asked him hours later. Of course I was mad, but he says at the time it happened when we had broken up and he didn't think we would get back together.

But the next day he was the one who texted me and called me up to fix things. Should I be mad that he slept with someone else and then the next day tried to fix things with me or am I just overreacting?