Hi I’m new here so sorry if I ramble. I’m not used to forums...
I have been with my partner for almost five years. We have a son together who was born in March this year, and we’ve never actually had any problems - or so I thought!
Whilst I was heavily pregnant with our son in Feb, I got a message on Instagram from some throwaway/fake account claiming that my partner was cheating on me, and claiming they could prove it.
Initially I ignored them but they wouldn’t stop messaging so I blocked them. They ended up making two more to message me, after blocking the second, I was getting annoyed, so when the third popped up, I simply answered “instead of just claiming you can prove it, ACTUALLY prove it. Just because you say you can, that doesn’t exactly make me believe you. Hard evidence please otherwise shut up.” They never replied, so I forgot all about it.
I guessed someone was just being cruel and trying to cause trouble! At 29 years old old myself I wasn’t having any of it!
Well.
That was until yesterday when my partner came home from work early, and I was of course at home with our son.
I asked “what have they sent you home for? Are you alright?” And he looked pale and shaky so I assumed he was ill, but he told me to sit down and kept saying over and over and over “I’m so sorry.” So I told him he was scaring me, what has he done?
He blurted it all out that back in May he had a one night stand with a new-ish colleague at his work, and now she’s claiming to be pregnant with his baby...
At this moment, I remembered the messages from February and brought them up to him. He said that was absolutely not true.She was only new at his company in May, and the night in question had been her ‘welcome aboard’ party, which I did actually know about, and I know that she arranged it all at the local pub. I have NEVER had any reasons not to trust him before now and he’s never treated me badly or lied to me before - that I’m aware of anyway! So I don’t think he’s lying about that. But then again maybe he is! Who knows anymore?! I asked him why he didn’t tell me about it before and he said because it was a massive mistake and that he regretted it so much and wanted to pretend it never happened and it would never happen again. He insists that they did use protection and she also claimed to be on the pill, so he doesn’t understand how this could have happened. He came home very early hours of the morning that night, I remember, and had the audacity to cuddle up to me in bed!!!! After having done the dirty with her!!!! At the time I just assumed that he’d ended up going back to one of his colleague’s houses to drink more after the pub shut! I feel like such an idiot! 😞
I told him to get some proof that she’s pregnant first, and he says he intends to. I asked what he’s going to do, and he said he doesn’t know because he doesn’t want her, he loves me, but clearly he doesn’t if he’d cheat! I said to him he doesn’t love me and he can't just try to force her to have a termination, as much as it kills me to say that, but it’s her choice and he’s the one who cheated! And he said “I do actually love you. But no I know I can’t force her. It would be easier though. But I know I can’t ask her to do that.” So I told him I need a break from him at the very least. We both cried, he kept apologising, but he ended up respecting my wishes (eventually!) and leaving, he’s stayed the night at his sister’s (confirmed by her). I haven’t slept. I just keep crying. I can’t believe that he’s done this. I really had no idea at all. No red flags, nothing! He’s never been nasty to me or cheated before, he’s never been manipulative or lazy or anything! I always thought we’d had a good sex life and that he’d always found me attractive - he said so! So why would he do this? Our relationship has always been amazing! Obviously not! What on Earth do I do now? How can I cope if she really is pregnant with his child and decides to keep it? What about our son?! Only four months old and already his Daddy has torn our family apart! He’s an amazing Dad to our son, and is very hands on. I just can’t believe he’s done this...
I’m in so much shock! Heartbroken. 
Sorry this is so long