What Causes Delay In Periods?

hi doctor..! I m 24 yrs old..and I have irregular periods..i.e my menstrual cycle gets completed within 20 days and so i'lI get periods thrice in two months.moreover...each time my period stats...it continues for about 10 days..with very little bleeding..on alternate days.I feel very inconvenient about this and cant concentrate on my regular activities.I hav got my abdominal scanning reports and thyroid test done two yrs back..which all indiate normalcy.I had been to a gynaecologist and used medication..during course of medication evrythng was set right..but my problem started again when the course of medication was over.I am soon getting married.Please help me to out of this problem.