16 years old (130lb). gain/lose weight really easily. can only maintain weight if i eat <800 cals. i've gained 20lb in a month before. normal?

Not normal: If you were a sparrow i would tell you that eating less than 800 calories per day was fine. By eating so little, your metabolic rate will drop and you will put on weight very easily. By limiting your self to few calories you will likely feel deprived which can lead to binging. Please tell your parents what is going on so you can get help from your pediatrician.