I'm no stranger to Beta fish sickness. Last year my fish got hit with PopEye, then Ick, and then Fin Rot all in a row, and after about six weeks of really really careful care, he survived! But the popeye scarred him and he's never been perfectly balanced since.
Now a year later, there is something wrong again, but he has no visible symptoms! The ONLY thing noticeable is that for about 3 weeks he's been seemingly unable to swim gracefully, he jerks around when he does move, and spends the rest of the time laying at the base of teh tank.
His colour is normal, he has no spots, his shape is the same, he is definitely not constipated,..
But now he cant eat, because he doesnt have enough control over his body to get the food into his mouth.