I always have my periods on time, sometimes before the due date but never after the date. Once or twice I got it 10 days before the due date and then in the next month I get it on 20th(the usual date), last month also I got it on the 15th and this month I waited till 15th but I didn't get and today is 24th but still I haven't got my periods, Early in the morning at about 3.30 am I did a pregnancy test but it was negative. Could you please tell me what must have happened and what are the possibilities and if I could be pregnant or no.