What Causes Irregular Periods Even After Having Medicines?

Hi, I am a thirteen year-old female. I started having my period when I was about 10.I've been having irregular periods. Some times it delays 3 months, 4, 5, sometimes 6. I went to go see a doctor a year ago about it and he gave me these pills that would make my period come. It did, but the irregulation continued afterwards. I've been looking at Secondary Amenorrhea and curious to see if I have it. Is it possible for a 13 yearold to have it? I also want to have kids when I get older but am worried at the thought of the condition. Would I be able to get pregnant if I have secondary amenorrhea? and is it possible to get my menstration cycle regular again?