My boyfriend says I’ve cheated

I have not cheated on my boyfriend, I’m 14 weeks pregnant yet he still accuses me and says he don’t want sex with me until he’s been checked out for STIs, I’m clean I’ve not cheated on him, I think maybe he slept with someone else and maybe now he’s thinking he needs to get checked for STIs, I do t know what to do, he says he hasn’t cheated but he did a few months ago!! How do I get the truth out of him, he’s making me ill saying I’ve cheated when I haven’t!!