Uh oh.

What would you do If your boyfriend of almost 4 years, hung out with another women while you broke up for a week, she says they had sex more then once he still denies it.. I take him back after that, then 5 months down the road you hear that chick is pregnant, and still single don’t know how far she is, he still denies it saying I have nothing to worry about... what would you do?? Help.