Missed Periods but no Pregnancy

Hi, Maria me again at your site for a question that's making me confuse. usually i had a monthly cycle of an average 28-32 days not more or less but now i m worried as i missed my periods, 20 days are increased even i made twice pregnancy test from an authorized labaratory but still its -ve. now is it normal or is due to any disease. i do not use any contraceptive or any other thing. i must tell that last month i had menses on my exact date but flow was very low and stopped within one an half day rest was spotting but that was also very rare i m having irregular menstruation from atleast 2 years and i m fed up of doctors as the just treat me as secondary infertile case n says normal to other things . 1 year before i also went for hysterosalpinogegraphy, it was also normal. please tell me what should i do now