What Causes A Light Menstrual Period And Negative Pregnancy Test?

hi i was wondering if it was possible that mirtazapine would give false negative pregnancy test result, i had a light short period whilst on them, did a pregnancy test which was negative,so then had marina inserted beginning of may,I stopped mirtazapine then, as i had put on a lot of weight and was sleeping alot unhappy with there side affects which i did around the first week of june. i feel pregnant and look it but test results are still negativehad the very light spotting periods for the first 6wks then practically nothing since which i understand to be normal.. please advise thanks in advance