What Causes Irregular Periods After Protected Intercourse?

i am 17 and i just skipped my period for a month but the next month had it normally. this last month, the week of my scheduled period, i noticed brown discharge and spotting in my underwear and when i wipe that lasted for about 5 days. i did not have a "normal" period like usual and thought it was just due to stress. about 2 weeks later i am experiencing the same discharge problems. i have had protected sexual intercourse in the past month am i pregnant?