I’ve been dating this guy for 5ish months. We met through work.
We see each other regularly and I’ve met his friends. He suggested we go on holiday a few weeks back and we’re meant to be going this Friday.
In the past he’s made “jokes” about other women being interested in him that we work with - just stupid stuff as well like his brother is coming to visit him to see what tinder is like where we live 
I told him he it was immature and stupid and he stopped doing it.
then the other night he was at my house. After we’d had sex I said something along the lines of how long it has been since I last had sex (as we’ve not seen each other for a week). He replied
him: “I had sex yesterday”
me: “oh right. Who with?”
him: “myself”
I literally couldn’t believe it, like what the fuck. I asked him to leave and he just said he was joking and he hasn’t been with anyone else. I still asked him to leave and text him saying I don’t want to go on holiday anymore. He keeps texting me saying it was a joke, he was telling me he’d had a wank and that’s it.
one friend told me if he was actually sleeping with someone else he wouldn’t risk making jokes about it & it sounds as if he’s trying to get a raise out of me.
im not too sure what to do really - I do like him, but I am insecure and the “jokes” don’t help. I feel awful that he’s lost all the holiday money, but I just feel like how can I go if I have even a slight suspicion he’s cheated??