Am i underweight or.....?

i went to the doctors today, and i weigh 87 pounds. im a 13 year old female, im also 4'8, yes i am short, but thats besides the point. i cant exactly say 'i have an eating disorder' because i havent been diagnosed with it, i eat about 800 calories, and my self esteem isnt exactly through the roof. of course, my doctor doesnt know that. she sent me to a nutritionist and said i was underweight, (at my last doctors visit i was 97 pounds) she said that for me, 100 pounds was normal. isnt 87 normal though, like average weight? another lady, when i was checking my weight, said it was to low too. but i checked online and everything, and 87 IS normal. did she just say it because i lost 10 pounds?