who could be the father?

I would say it's not the x's baby. Ovulation usually starts about 14 days after the first day of your last period. So, if you had unprotected sex with your current bf a week after you period stopped, that would put you some where around your ovulation time. If you weren't ovulating exactly right at that time, give or take a day here and there, sperm can survive in the body for a few days after ejaculation as well.
So...if you had sex even up to 3 days before you actually began to ovulate, the living sperm from the previous ejaculation can make there way to fertilize your egg. I would be safe to say that this baby is probably your current mans, tho this is just my opinion.