Pregnancy

hi po! kailan po magstart ng bilang ng pregnancy? my last menstruation po is feb 15 and hindi padin po ako nagkakamenstruation. me and my husband were suspecting na pregnant na ako pero nagPT po ako ngayon and it was negative. for the last two months sunasakit puson ko and pati dede ko po para namamaga. maari po bang mali yung PT?