Why Are My Periods Delayed In Spite Of Taking Medicines?

hi, m 22yrs and my periods hv been irregular. last month i had my periods 10 days erly, and thsi month it ws delayed for more than 20 days. so i consulted doctor and she prescribed me primolut nor 10 mg norethisterone acetate. she gave me 5 tablets and told me to have 1 each day and if my periods starts thn will hv to stop taking ths. Today saw spot tings but the periods are not like i used to have it before. its just a spoting. Can Any1 guide me whts wrong.