What Causes Spotting In The Mid Of The Menstruation Cycle?

i had a normal period on 5-6-2011 bleed for 4days which is normal...now today 5-16 i went to the restroom wiped and notice blood on the tissue but none on underwear...went to get tampoon and when i came ack blood is now brown like when you go off period...not on birth control and my periods are normal...could i be pregnant?