Late period

I need some advice, I’m currently dating someone, we got into a relationship on 1st December. Prior to this I was sleeping with someone else. I’m 3 days late for my period. I last slept with the other guy on 29th September. I’ve had a period (or what I thought was a period) since then. And I slept with my now boyfriend for the first time 27th November. Now my boyfriend has told me that for medical reasons he had an operation that means he can’t get anyone pregnant. The other thing is, when me and my boyfriend slept together, I was 7 days past ovulation. When I slept with the other guy I was 1 day past ovulation. Does this mean that if I am pregnant it would be the other guys?

Thanks for any advice