Help with spotted cory

[Seaman]

one of my 4 spotted cory's turned black...his belly isnt sunken in like someone told me to watch for and he is acting and feeding normal from what i can tell. Also its been this way for probbably 6-7 days...

heres the before picture...

heres the after picture.....

 

[Seaman]

Good news his color is returning to normal.....I am completely stumped as to why it would do that.......