What causes delayed periods in an 18-year-old?

Hello doctor,

I have been having some problems lately with my periods. I am 18 years old and I got my first period at age 11. Since then, they have always been irregular, my cycle would last about two months (sometimes more sometimes less). I think it was during my first years when I once did not get my period for like four months but when I told a doctor, they said this would be normal in the first months. But now I got my last period once in the last three months. I have not had sexual relations so that's not the problem. I am also concerned about the fact that two months ago I had a brown discharge for like five days and now it is back again. This is my fourth day with a very very light brown discharge but that continues daily. I would like to know if I should be concerned or at least understand why this is happening.