Hey girl dont stress when I started my birth control I was having the same thing and of course I was freaking out just talk to your doctor mine changed my BC and I never had the probelm again
If you've been on BC for 20 days that means you are about to the period pills. The brown discharge may be the start of your period and the stomach pains could be cramps. If you were pregnant you would only have been pregnant for 8 days, too soon to be having symptoms. I doubt it's pregnancy. Don't stress though or it will make your period late, causing you to stress more. You should go get a home pregnancy test to ease your mind and just wait for your period. It's a good idea to use condoms, even if you are on the pill.
I would wait for your period to be sure, but I had something similar happen to me after I had sex with my bf for the first time. If sex was painful, it might just be a discharge from some possile bleeding, which would also account for the stomache pains. I would strongly recommend using a condom as a 'plan B' for this, but if the sex was slightly painful it might just be from the penetration.