I'm currently 28 weeks pregnant. On October 11, I cheated & the guy didn't use a condom. My period was always very irregular to the point where some months I didn't even get my period. I think I had my period in October but I'm not 100% sure.. but on November 21, I had my last period. On December 19, I found out that I was about 4 weeks pregnant. Is it possible that it could be the other guys baby? Please, any help or advice is welcomed. I just need some peace of mind.

It is very unlikely this other guy is the father. I would not worry if I were you.

If you had your period after you had sex with the other guy then NO you definitely are not pregnant by him.