The muscles that largely move your fingers are in the lower arm. So this probably isn’t a muscle issue.
Probably nerve or joint damage. You mentioned “equipment.” Do you use heavy equipment that vibrates? You may have bruised the cartilidge on one side of that joint. Or caused trauma to a nerve on the side that hurts.
Or it could be something else entirely.
Short version: You need an xray of the joint at issue.
I do a lot of data entry