Well, you got lucky. Because I was very thin, almost anorexic in my childhood. And I can tell a bit.
Recently, I gained weight unintentionally during COVID and with constant feeding from my coworkers so I fattened up quite quickly.
From what I can describe for you. At the beginning nothing will change for you, until you feel that your clothes starting to get smaller and tighter on you, then you could feel your thighs rubbing each other while standing, then you notice your belly getting snug and bigger.
If you continue to gain weight more and more like me, with stuffing yourself silly and eating bigger portions throughout the day, you will feel more things making you realise you become fat. Like, you will be hungrier, as you continue to grow your belly will stuck on the way, while trying to wear shoes, your wardrobe will be smaller, pants becoming less comfortable and your underwear will started to roll under your growing belly.
In time you get more rolls on your body, your chin will double and you feel much more sweaty than you used to be. Much more.
also, in my case I gotten rolls in places that I never had them before. If you balloon yourself to enormous size you will get back rolls and fat all over your body. Your belly will get stretch marks.
And also, it will be harder to walk, especially on stairs. You will get very lazy.
As you continue to grow, you might develop get breath issues as you will need to adjust your body to the new added weight.
And that just the things you might feel, there is also other issues regarding the fatness that will affect your body.
11 months