Why I Am Not Getting My Period?

I know this can happen from being underweight, but i am not. I used to be a little underweight (BMI 18.1) but now I am 131 or 132 lbs, which means my BMI is 18.5 or 18.7, which is normal. I have been this weight for a few months now. I am also extremely tired all the time. What is wrong with me? I used to have sooo much energy. Now, i can t even play sports anymore. And why am i not getting my period? I haven t gotten it in almost a year. Also, i am never hungry. I have to force myself to eat because i get full so easily. And i am very constipated. However, i do eat a lot to make sure i do not lose any weight. I am not pregnant because i haven t had sex. I m a teenager. And yes, i have had my period in the past.