Go and see your doctor

Posting as a layman.
I developed tendonitis as a runner 15 years ago.
Stiff or sore tendon in the morning, crepitus feeling in the tendon.
The advice from a physio was friction massage on the tendon.
Finger and thumb massage each side of the tendon for 10 minutes in the morning and evening.
Friction massage stimulates blood supply to the area.
Do the massage on the most painful area.
Repeat daily until pain free.

The tendon has a poor blood supply.
Swimming is good exercise and easy on tendons.