How are you extremely skinny? Do you have a digestive issue or something?
I struggled with weight gain for many years due to crippling IBS but I found some relief while bulking from a certain probiotic. (Got lucky - dozens of things I tried including fodmap, etc. never worked. That said, the probiotic only helps - not solves... and doesn't solve my absorption issues)
Surgery, steroids, and a serious bulk are in order if you're serious.
Great question! No medically diagosed/diagnosable condition for better or for worse; someone's got to represent the 3σ end of the normal distribution right?
I can make the pounds go up or down in a few weeks by eating more or less, but it goes mostly to the gut (Skinny fat = probably the worst look), and just enough to pad the arm muscles so it looks like I've never been to the gym in my life! Lately have just been embracing it and staying lean (Looks like 12% maybe?) while working out as much as I can stand.
I was about to do the steroids... (low dose test) I suspect that would genuinely fix it. But when I got the blood test back, test was smack in the middle of the normal range! So would have to shop around a bit, and trying not to lose my hair. But may still do it. A bulk counter-intuitively is not the move because it would just put me back in skinny-fat land.