Need some help on this one

Okay so yesterday and today I had some cramping, I normally come on my period either Wednesday or Thursday & I'm also on the pill. I was taking antibiotics my first week and my bc, and I finished the pack, but I had unprotected sex throughout that time. Before I got the antibiotics the doctor asked if I was taking anything already and I said just bc and she said okay and put me on amoxicillin, with some added ingredient. So today I had more cramping and I even started to see some blood, but it wasn't a lot. So I put a tampon on at 4:30pm today, now I have been laying down for the last 5hrs watching Netflix but when I went to change my tampon about 20mins ago, there was barely anything on the tampon, not really red, mostly brown too. Is this my period? Is there a chance of pregnancy? Has this happened to anyone?