Questions...

Heather • MSW grad student. Ttc baby #1. Want to have a boy and plan it out so that I`m on break from school during the due date!
We just started ttc baby #1. I'm a little confused because of a few things though and was wondering if someone could help me out. 
I'm 24 and was regular. I've had a period every month for forever the only thing is I never really kept up with it or tracked it. I don't even know if it was time for AF because I never really kept track of my period. We just wanted a baby and tried and then I started to track on this app in case it wasn't the right time so that we could figure out when it is. I'm overweight so idk if that may be contributing.
A few days after having sex I felt a little weird in my lower abdomen area and was thinking maybe I was preggers. Then I went to the bathroom and noticed a little spotting. I had heard that was normal so I thought maybe we still had a possibility but the next morning it was more than just spotting though not quite as much as I normally experience during AF. 
It's been about 3 days into what I thought was AF but I have been like SUPER SUPER light compared to my normal flow. I only had pain and back cramps the first day for part of the day. Normally my flow is really heavy and I have cramps and backache throughout AF. I'm confused why it would change so drastically. Any thoughts?