Not sure what is wrong with my beta fish?

He hasnt been eating and doesnt pay attention to me like he usuallly does..all he wants to do is hide in his shell. He only comes up for air real quick and has an attitude about him...When when something touches him or a plant rubs against him as he swims past sometimes he will freak out like swim away in a flash. If hes not hiding in his shell he just goes to a corner of the tank on the bottom and trys to burrow himself...? Has this happened to you? He seems to be swimming just fine and hes been like this for like 4 days..I would have thought he would have died already if there was something seriously wrong with him just dont know why hes being like this?