Unsure of my baby’s father! Help!

Patient: I found out I am pregnant today and assuming about 4 weeks. I had sex in December with my ex on the 23rd three days after my period.. I then had my period on January 9th. I have has sex with my previous boyfriend around my assumed ovulation date and am now pregnant. Is there anyway it could be my ex’s baby and the period could have been something else?! I am so nervous.