What Is Causing Me To Feel So Weak?

I'm so weak, physically and mentally. Just getting out of bed or walking down the stairs is a massive problem for me. I'm seventeen, and I have a history of eating disorders, but I have been eating regularly and healthily for the past six months and my BMI is currently around 18. I get a lot of sleep, usually between seven and nine hours, but when I wake up, I just feel drained. This is interfering greatly with my general functioning. I miss a lot of school and rarely leave the house because it makes me very weary. I feel like I need a lot of time to be alone and I am crying a lot. I haven't have my periods for quite a while and I am mildly anaemic. My gums have gone white. What are some possible reasons for why I feel so weak and drained, based on the symptoms that I have provided?