i think im pregnant

hi im 20 and my bf is a marine in the military and when hes at home on his leaves we will have sex using the pull out method almost every night/morning when hes home he left on may 3 2013 and in the last week iv been having back pains headachs and stumach pains and sometimes diarreaha. i took two pregnancy test and both said i was negative i dont get my period tell the first week of every month. are these signs of pregnancy anyone.