what can cause an abnormally fast metabolism

I seem to have a very very fast metabolism. What can cause this and is there any problem with it?

I am 26 years old and I have had to eat more than other people for as long as I remember. My mother asked my doctor about it many times growing up but no cause was ever found.

Recently I had my RMR measured and the result was 4350 cals/day. I eat around 7000-8000 calories a day.

People tell me I must not have prepared for the RMR test correctly and that I can not really eat that much etc. but I did prepare for the test correctly (12 hour fast, no exercise, no caffeine, relaxed and calm during the test etc.). My calorie estimate is from keeping track of everything I ate for a week and making my best guess at the number of calories from the information at colorieking.com.

I am not a ‘compulsive over-eater’ or something. I am eating what feels right for me.

How common it this? What can cause it? and is there a problem with it?