Brown discharge instead of Periods

Hey Girls...I need help! I had protected sex (using condom only) on 31rst Dec and the 13th Jan. My periods were due on 19th Jan but instead I can notice a brown sticky discharge. No period. Little cramping and lower backache. Took two pregnancy test and both are negative (one before period was due and one today). Let me know what is happening? What is this brown discharge? Could I be pregnant?