It is best to go to your doctor and explain all of this. If they feel you need to see a specialist again then they will refer you. If you have been irregular before and now they have stopped which could mean something is not right. Even at a young age. Always best to be checked out!
A doctor really needs to diagnose the problem. ...Could be a number of things.
Myself, I have something called polycystic ovarian syndrome or PCOS. PCOS presents mainly with irregular periods. There are other symptoms as well. A doctor taking a comprehensive history, along with blood work, physical exam, and pelvic ultrasound, can diagnose PCOS.
Although my cycles are now regular (27-31 days) (as you know, "normal" is every 28 days), most of my life I only had 1-3 periods per year. I have no other PCOS symptoms besides the (formerly) irregular periods.
My PCOS has had no ill effects on my health that I know of (I'm almost 50). PCOS can make it more difficult to become pregnant for some. Some PCOS women have more severe symptoms including head hair thinning, excessive body hair growth, acne, oily skin, large pores, and obesity.