Opinions please? Has he cheated?

I've been with my boyfriend for 4 years. We got engaged a couple of months ago and the night we announced it on Facebook this girl that was friends with him on there deleted him then added me. I asked him what it was about and he said he didn't know and she was a bit weird but I just accepted anyway. I'm 5 months pregnant also.
Anyway she had made strange random comments on my photos and I asked him if anything had ever happened with her or what her issue was and he said he didn't know.
Tonight I got a message from her saying "he cheats on you" and basically she had screen shots of her saying to him tell her or I will and he said what's the point I haven't cheated it's in the past leave it.
So it turns out that he had kissed her (he says that's all) a year before we met - and that when we were having trouble a couple of years ago he messaged her a few times for someone to talk to? (He moved 400 miles to be with me and she still lives 5 hours away) and she seemed to have it in her head he's lying to me and kept repeating poor me etc.. options please? My heads a mess and I can't eat as I feel too sick my poor baby must be staving.