6 weeks and counting of constant bleeding -- only overnight!!!

Okay, so, I was on both the depo shot and the pill. The depo shot made me horribly sick, so I stopped taking it and began taking the pill before the shot protection ran out, hoping the pill would rush the depo out of my system. Yeah, huge mistake. I was so ill for the entire month I took the pill with the depo, that once the pack was done, I just avoided it and decided once I got my period, I'd go back to natural family planning. Yes, I already have three children, and I'm 25, and while I MAY want one more child down the road, I just am not ready at the moment. I'm in college, and I have goals to lose my baby weight from my pregnancies, so, I really need to be able to track what is happening with my cycle, because I would definitely still like to be sexually active, and, I'm sorry, but I don't out all of my trust and reliability in  condoms. I just don't. So, the weirdest part of the bleeding that I've been having since I stopped birth control? Yeah, I only bleed overnight. I have had a few days where it gushed a little bit in the middle of the day, but that was for maybe 2 days out of these 6 weeks. I'm beyond frustrated, and am not too sure if anyone else has experienced something like this? My OBGYN simply said stay off of any type of contraceptives for 3 months at LEAST, but, how am I supposed to know when I'm fertile and when I'm not?! Why do I only bleed at night?!?! Ugh. Any information or similar experiences would be fantastic.