@ruthclayton6 Ouch! I have inflammatory arthritis, osteoarthritis and have had tendinitis and several hand surgeries, so I can sympathize.
First, let's start with the obvious - find another way to manage your phone when you run - pocket, fanny pack, armband...this will remove one stressor. Also, it's good for your hands and wrists to limit the amount of time you spend scrolling and texting on your phone - those tiny devices can cause an amazing amount of pain.

Second, I was the ergo-maniac for my staff years ago when people transitioned to spending hours at the keyboard every day. Tendinitis, carpal tunnel syndrome, neck pain and many other woes were rampant. I
worked hard to be sure my employees were using correct posture & taking breaks.

To get to the root cause, you need to look closely at your phone and keyboard use as well as piano playing. Make sure you are using proper posture and body mechanics and taking frequent breaks (literally every 20-30 minutes) to flex and stretch your fingers, wrists, arms, shoulders, neck and back.

A visit or two with a hand therapist (usually an specialized occupational therapist) for evaluation can help. Also, there are exercises and braces that may help you. And Kinesio-taping for day long use. All of these are tools that a hand therapist has.

During an acute flare, you might be advised try anti-inflammatories - ibuprofen if you tolerate it or topical voltaren gel. You can also try icing, but it makes some people's hands feel worse.

Have you seen anyone about this issue?