For the last 4 months i've had an extremely heavy period with very large blood clotts, due to this i dread getting my period. however this month it's suddenly a great deal lighter, what could be the reason?

Normal variation: Menstrual cycles can fluctuate from month to month depending on hormone fluctuation caused by stress, weight loss/gain, exercise, and lack of sleep. It is not usually a concern if this happens once or twice. If your cycles cause increased bleeding or you stop bleeding you may want to talk to you Dr about the situation and have a thorough exam and lab tests.