What could be some reasons i'm experiencing sporatic severe hot flashes when i'm 20 and only 102lbs?

Low body fat?: Some lean athletic women don't menstruate because of low body fat. Assuming you are of normal height your body weight puts you in a lean category. You could have low estrogen levels. If you aren't having a regular period you should get a check up with your gynecologist. Sometimes early thyroid disease can trigger changes in estrogen as well and this should be checked.