Hello! My right hand, middle finger Distal Interphalangeal joint. This joint feels tender when I press down on it, then straighten it. What?

Arthritis synovitis. can give a slight bit of swelling in the joint such that fluid when it's moved around takes up space and you interpret that as discomfort or stiffness. Even very early arthritis where the X-ray might seem normal can have this Synovitis can be swelling alone. Most common is OA. But many varieties of arthritides exist including associated with celiac, gi issues, infectious, rheumatoid like etc.