I need words of wisdom please.

My boyfriend and I have been dating for 8months. Early in the relationship I found out he was texting his ex girlfriend while we were on a date. We got at it and I was really mad because why is he texting this bitch who he said was a jerk to him?? What business do they have pending?? He had mentioned that she treated him like shit, she still has feelings for him, that she was lazy and she took advantage of him while in their relationship. OK so, I got mad and brought it to his attention. We had a fight over it and I gave him an ultimatum I would say. I said block her or I'm leaving you. He blocked her. We were OK until Sunday she had messaged him and it turns out they have been talking again. He keeps telling me she's the one that messages him and that he answers her...he lied to me because he unblocked her and is still talking to her. What gets me even mad is I haven't heard good things about her. I've heard she gets around and will try to get at a man no matter if they are married or taken. I know it's true because I actually work with one of her old friends and she told me. OK so back to Sunday, she sent "we'll find a time to go" and he swiped the text quickly out the way... I guess hoping I didn't see... What would pop in your head ladies My first instinct is he's cheating on me and he lied!!! Where are they going?? Why??? My boyfriend was like "she has a bf now and she wanted to go on a double date".... Ladies is that even appropriate?? Knowing I don't like that women?? Is it a invite for me to go kick her ass???I'm so frustrated still. He's coming over in a few hours and I either end it or he takes that women out of his life. Am I over reacting??? He tells me he is not cheating on me and that he doesn't block people because there are no intentions on getting back with each other and so forth.. He lied to me.. How can I belive him?? What do I do?