not sure what to do or feel

my fiance likes to hang out with coworkers until late in the morning. which is no big deal to a point. I'm four weeks postpartum. I just found out he's been hanging out with a female coworker 5-6 times in the past couple of weeks. he sees this as no big deal. I haven't met her and know nothing about her. should I be worried or am I just blowing this out of proportion. I get that males and females can have platonic relationships. but to hide one for weeks makes me suspicious of the whole thing