Weight distribution

I want to gain weight. I have a fast metabolism but whenever I gain weight I gain 3 or 4 lbs then lose it and repeat. Currently and whenever I do gain weight the weight go to my upper body under my armpits making me look very heavy at the top. That skin that be poking out when you put on your bra, that’s what I’m talking about. I’m very petite and thin and have small narrow hips and broad shoulders already. So it make me look really really heavy at the top and I don’t like it and I’m very insecure about it. Then I have small thin legs. I just want to tone my underarm so it don’t look heavy. But I’m wanting to gain weight but I’m scared that it’s going to go where I don’t want it.