Betta Fish: Help!! My beta keeps sinking down

He was okay till yesterday today I started noticing him sitting down on the gravel in a weird way with his fins all cramped up and spine all bendy.. he swims for a few seconds then again ends up sitting like that.. as though his tail feels heavy.. what do I do.. I took him out in this mug to check for damage or any signs of parasites or infection but he seems fine.. just not able to swim.. he has no tank mates.. he had a bloated belly a few days ago so I fasted him for 2-3 days and the swelling went down.. I’ve been feeding him quite less since then.. I’m new to this