brown discharge/spotting

Kelsey
So I made the rookie mistake of having sex the week right after my latest period ended. The thing is, and this is tmi so you can skip ahead if you wish, it was sort of his first time being on top and we couldn't find the right angle so we did what we knew we could and switched to anal. During which he continued to do so after he took off the condom. Now I know I can't get pregnant from anal sex but he would slip out and basically miss and go in the slightest bit into my vagina. 
So I guess my question is, before I completely freak out is there any chance I could be pregnant? I am on birth control and was at the time. However recently I've been experiencing frequent stomach aches that feel really different from any other I've experienced. It's unlike period cramps or anything I've ever had. And just today when I came home I noticed that I had the slightest of drown discharge. I looked it up on the <a href="https://play.google.com/store/apps/details?id=com.glow.android.eve">eve</a> app and all I could find is that I could be pregnant. Please if anyone could give me any information or anything that could ease my mind.
Update the drown discharge i don't think has continued but rather there is now the smallest amount of red to witch I hope is my period