How many calories should i consume a day?

I am 13. 5'2" 1/2 and around 112-115 lbs. I have Female Athletes Triad. i got my period when i was 11 and my periods were normal for about 2 years. Then i started unhealthy dieting when i was 125 lbs and got down to 105 lbs. I didnt notice my period gone untill after i stopped my strict dieting..and i lost the weight FAST. i was only eating like 500 cals and i also was in track where we ran 3 miles during practice. Anyways I try to eat around 1800 cals a day now, and 2000 on REALLY actice days (when i swim for 5 hours with my friends) and i gained a pound :(..and yesterday i saw white discharge! (sorry, TMI) and i only ate around 800 cals yesterday ( i no i was busy at Six Flags, and we walked ALOT so i lost alot of cals doing that) and the discharge is gone :(. now, idk if i should eat alot today to make up for yesterday, or normal amounnt. and also, how many calories should i eat on a day i DONT excercise (i checked my BMR and it says around 1200-1300 calories, but that seems WAY to small) alls i want is my period back and for me to mantain my currrent weight.