My girlfriend and i have been having sexual intercourse for a while now, but last period she's had was 3rd of august, we always use condom, but sometimes i start without because with condom it hurts her, but i ensure that after the initial penetration i put it on. two weeks or three ago she made a pregnancy test which was negative and two weeks ago had an implant put on her arm, still no period to this day and she's gained some wheight, could she be pregnant or is it because of stress, implant?