Is My Betta Sick?

[Samsonite959]

I just got a new betta about a week ago.
I changed his water recently, and now it seems like he's acting kind of weird.
He swims around a little faster like hes trying to get something off him.
Maybe I just didn't notice before.
But also, it seems the the bottom small, skinny fin under his body is rotting off??
I don't know if it really is rotting or not.
The bottom of it is a white.
Just the tip of the fin.
And I can't tell for sure if its getting any shorter.
But it appears that there possibly are peices of it at the bottom of the bowl.
Although the peices are brown.. and possibly skinnier, so I don't know what those are.
He still eats though.

In short, I don't know if he's sick.
Could anyone tell me if this sounds like an illness, whether its fin rot or something else??

 

[SPLiSH]

The swimming faster sounds okay, unless he is rubbing up against things it doesn't sound too odd.

Could you maybe get a pic of this rotting fin and the bits on the bottom of the bowl? The bits may in fact just be fish poo.

See if you can get some aquarium salt, and give him some. Also, keep his tank warm and the water clean.

Keep us posted!