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You have tendonitis, not arthritis. Google 'Trigger Finger' for more information. Your best course of action after becoming more informed about this condition is to see your Doctor. It's usually very treatable.

First of all get competent medical diagnosis with ensuing action.

As a temporary relief you may try to arch your fingers more or less to prevent the thick point passing a narrow point. Which degree of arching or straightness to apply can only be determined by experimentation.

Klaus Smedegaard Bjerre

I had a similar problem a couple of years ago playing bass guitar. I use a 6-string bass with an extended scale length, and it's a bit more demanding physically than the average bass. After a couple of long gigs in a row, I began feeling the tendons on both middle fingers stiffening and pulling. It lasted about a month. I kept gently moving and stretching; letting them rest and set up made it worse. Occasional use of a heating pad helped, too. I have an occasional flare-up, but it hasn't bothered me in a while.

Still, every case is different. It's best to see a hand specialist if it persists.