I was thin as a drain pipe in my teens and twenties. I was also so body
conscious I wore clothes a size too big, I hated my bosom and did every
thing I could think of to hide it. I never dieted or thought
about calories.
Stroll on middle age. I just look at a peanut and put on a elephant. Lol
I'm no where near as body conscious any more. Im currently on a diet
trying to get fit and slim and I can't wait to be able to wear clothes a size
smaller!
Unless you are unhealthy or have been advised medically to put on weight,
I wouldn't worry about it. Instead why not try some weight training to
build muscle.