I had sex with my ex dec 15 and then six days later around the 19 or 20 I had a period and then I had sex with my boyfriend around after christmas and now I'm pregnant due in sept 24 who can be the father

With dates so close, either man could be the father. You will have to have a paternity test done after the baby is born.

Just too close to tell, any answer would be a guess. When the child is born you will have to do a DNA test to be sure