I've been trying to figure out this pregnancy, can someone help?

[Eieen]

I started my last menstrual October 12th and it ended on October 16th.. The last time I had sex with my current boyfriend at the time was October 23rd. I took a pregnant test November 13th. It was negative. I started having sex with an ex boyfriend. I took another test November 24th and it came out to be positive.. Who would I be pregnant by because I'm not understanding. Thank you