You need composite evaluation of the cause... Need sonography and hormonal evaluation... Get expert advice

hello, have you gained a lot of weight? If yes then irregular periods and sudden weight gain are common signs of PCOS. Visit a gynecologist and get yourself evaluated for PCOS. She will prescribe you medications to regularize your periods.

You will need to do a few tests to diagnose the problem. You might be having PCOS or hypothyroidism. I don't know your weight do it's difficult to say for sure. If you have other symptoms like weight gain, hair growth on face, chest, abdomen, it may be PCOS which is quite common in young women. If you are sexually active, a urine pregnancy test had to be done first