Super irregular periods and lots of pain

I first got my period in 6th grade, and I am now entering my senior year in high school. My periods have never been regular, skipping whole months and sometimes coming every other week. But now, I am going the longest I’ve gone without my period. I had it last at the beginning of June, and it is entering the end of August. I have pain every day and my uterus area is tender if it gets poked even just a little bit. I am so nervous about what this could be. I have taken multiple pregnancy tests and they all have come out negative and I have a doctors appointment coming up soon. What should I expect to get told? I have pretty bad cramps when I do get my period, my periods are super heavy and usually only last 4 days, and I did start bleeding for only one day two weeks ago. I don’t experience pain during sex and I have been getting dizzy and nauseous when I go to bed at night. Any ideas?