I noticed that after making a fist, when I try to straighten one of my fingers, it catches and becomes very painful. Sometimes it is necessary to take my other hand to force the finger back into a straight position.
The condition you are describing may be trigger finger (or trigger thumb), and is frequently caused by overuse. Some examples of activities that might initiate this condition are power washing a deck for several hours, using a rivet gun repetitively, leash training a dog or opening window latches that have a lot of resistance.
Why is there a hard nodule present in my palm? It’s tender to touch!
The nodule is actually extreme thickening of the tendon, and each time your tendon “triggers,” there is an inflammatory response that occurs (see diagrams below). Look at the swollen tendon in the first diagram, then take a look at the pulley in the second diagram. You will see that at some point, the nodule becomes so inflamed the tendon can’t glide underneath the pulley — that’s why it “triggers.”