I'm not sure if my baby is my boyfriend's...

So I was trying to get pregnant with my bf and 3 days after my period i cheated and had sex with an ex...then the next week after my follicle check, i started using OPKs like the doctor said and me and the bf had sex (on the day I got a positive opk). I'm 31+3 currently and due Jan 16th (but now measuring a week ahead) It has me really worried that this baby isn't my bfs....

**I got my positive opk on April 19 and my faint positive pregnancy test April 29th