Used to have chubby cheeks at 14 my face fat disappeared opinions?

To add to that I've been skinny all my life but still had my cheeks,out of nowhere they completely disappeared. I noticed I developed an underbite during this time. Could that maybe be a reason? I'm 19 now and gained 20 lbs, and my cheeks look fuller but the sides of my face still look flat creating that oval look still. I've noticed that I've gained a lot of fat in my neck area instead, So again please offer me your opinions it's bothered me for a while. P.s I'm 5"4 140 lbs male