Everyone is different as to when they get their period after giving birth, but if you have had unprotected sex, then there is always that possibility that you could be pregnant as well. I would say that the only way to know for sure is to take a test, but since you have no way of knowing when you ovulated, the test may not be completely accurate until at least 2 weeks from the last time you had sex.
If you are really worried I would contact your Dr. Best of luck to you!
I had a constant period for a year after I had my DD but I was on "the shot" from the day after she was born, then I had a m/c and went MONTHS w/o a period... with DS my period didn't come back until he was almost 7 months old... but I was breastfeeding... sorry I couldn't be of more help.