Fluctuating weight

I'm 18 yrs old and this year I've been up and down in weight 89-117 lbs. I go through phases of nausea for weeks/ months at a time not eating much. I don't have an appetite but I feel full if I eat only once. The sight and smell of food makes me not want to eat. Then there's a phase where I'm constantly hungry and eating. I've been having a few health issues, in which my doctor cannot pinpoint or diagnose. I do know that I've had ulcers on my ovaries. Only 2 periods this year. I'm always tired and sleepy. I'm often dizzy and lightheaded. Something is not right. Can anyone relate? If you have gotten over it, what helped? I'm starting to get worried because it's almost a year, and I don't know what's wrong.