What Causes Brown Discharge During Periods?

To whom it may concern,
I take birth control regularly and normally get my period on the last Monday of each month, almost every month. I haven't missed a pill and I take it at the exact same time each morning. The day before I was supposed to get my period (Sunday) I went snowboarding for the first time, which involves a lot of exercise. I didn't get my period on Monday and was freaking out, scared of a pregnancy. I didn't get my period until Tuesday and there was only a little red blood at first. Now my tampons have been with brown blood instead of red. If I don't have a tampon in I do not leak on a pad. Am I pregnant or what would cause this?