Is having shorter periods after unprotected sex a sign of pregnancy?

[Maree]

i had unprotected sex with my boyfriend and he came in me and a few days after we had sex i started spotting and ive never spotted before and then my period only lasted like 3.5 days and they normally last 6-7, and its been more than a day since ive bled at all and now im kinda getting a little bit of brown discharge...could i be pregnant? and also my lower back is kinda hurting a little bit and ive been having headaches