Well my dear friend, if you did not have any sex after last period, you have to very rightly set aside the consideration of
pregnancy.
Hormonal imbalance can be the cause in your case.
I would suggest you to take some
progesterone pill (with the advice of Gynaecologist) for 4 days and then wait for menses to appear. You should expect withdrawal bleeding within 4-7 days. After that, from day 5 onwards start cyclical birth control pills for three cycles. If you are taking 21 pills pack, after finishing each pack, wait for the withdrawal bleeding to appear then you may start the next pack again from day 2-5 onwards. If you start on day 2, it will give you birth control effect also. If you are taking 28 pills pack, just go on taking the pills continuously. During last 7 tablets, you will get the menses. Usually, after 3 cycles, your menses should get regularized.
However, before starting these medicines, please consult your Gynaecologist to rule out any other condition and also to have suitable brand and combination of birth control pills.
If the problem persists, you may have to undergo some other investigations like
ultrasonography, blood hormonal profile, etc.