Confusion!

I’ve had my normal heavy period for the month of August, I had sex before my period but it still came on time and a regular heavy flow. I am on birth control pills. I am now experiencing constipation and Lower abdomen pain and feel like my boobs could be a bit heavier than normal.. I really don’t think it’s pregnancy but I’m scared to go to the doctor incase it is I’m only 18 !! 😢