I am on Birth Control and I recently had a bladder infection so I had to take antibiotics for ten days. I did not have intercourse for that amount of time. I did however have intercourse that month and I took one of my pills late and used a condom. Now i had what i thought was my period on the placebo days but it has only last three days and was lighter than normal. Is this normal? Are the antibiotics affecting me? I have also had some stress and rigorous exercise. Thank you.