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. 
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?