Betta Pectoral Fin Not Working On One Side

[Amunet]

I've been noticing one of my bettas resting alot today and he's just not swimming around.
But I noticed that his pectoral fin on his left side isn't working. It's plastered against his body. It's the same side where his gill does't flare out.
He'll come up to eat, but will make his way back to the bottom to rest.
What could possibly be wrong?
Nothing else seems to be wrong with him cept for the low energy and fin though.

EDIT:
Did a 80% water change, also put a little bit of aquarium salt in his tank. He's swimming around some and his fin moved some, but only a little bit before he put it up against his body again. Looks like he hurt himself somehow. Maybe burned himself on the heater.. even though I don't see any marks on him.

 

[WILDER]

Do you mean he's fin clamping as thats a sign of stress and desease.
I think I would add a bacterial med if he's listless and has no energy.

 

[Amunet]

Only that fin was clamped but he's doing MUCH better now. I think he was just stressed from him hurting it.
The only thing in his tank he could've hurt himself on would be the heater. He must've lingered near it for too long or something and it burned him.

He's been swimming around like normal and using that fin as if nothing happened.
I'm just glad it was nothing serious!