TMI!! Postpartum Sex & Infection Question

So with my first baby I tore and had barley any stitches and I stopped bleeding at 3 weeks postpartum and we had intercourse at about 3 1/2 weeks without any problems. This go around I stopped bleeding at about 2 weeks and we had intercourse at 2 1/2 weeks so at about 2-3 days of no bleeding we tried for the first time again (I didnt have any tears etc) even though I have a prolapse the Dr said when the bleeding stopped we could go ahead and have sex. Well the second day we had sex I had a lot of like brownish blood/discharge after when I wiped but only then. Nothing since but the tiniest of the tiny dot of brown discharge when I wipe only here and there but most times nothing at all. I have heard brown is old blood from my Dr from past events so I'm assuming that's what it was since I haven't bled anymore or red. That being said I've been googling trying to find out what vaginal infections you can get during postpartum sex and have not been able to find out what type of infection you can get just that you could get one. So I'm acting here. When they say you could get an infection from postpartum sex is it just like yeast infection, bacterial infections and bladder infection or what else and how would you know other then having your Dr check.?