I will ask the obvious and say did you add aquarium salt in the water change.
However, black spot disease is an affliction of fresh water fish, not usually bettas, but it can be. It usually goes away on its own, but removal to a quarantine tank, tank water change-out, etc. can't hurt.
If you want to treat "black spot" disease, use formalin, which is very strong! Use gloves, follow directions EXACTLY.
The scientific name is Posthodiplostomum, and formalin should only be used in a hospital tank at about 75 ppm. And even then, only a one hour exposure.
I would personally just do a QT, water change, salt like usual.