Are you sexually active, if so then do urine pregnancy test. Were your periods regular before this?
It can be because of hormonal imbalance. Get your thyroid profile, cbc, fsh, prolactin, blood sugar fasting checked from some lab.
If everything is normal then do an ultrasound of abdomen.
Homeopathy is a very effective for this problem n has very encouraging results. And homeopathic medicines are safe and do not have any side effects at all.