I've got tennis elbow in my left elbow and it's quite painful.

I suspect the cause is a combination of knitting, weeding, typing and concertina playing, with knitting probably as the main culprit.  Knitting was a new activity for me - I've done a fair bit of knitting recently and carried on even though I was hurting because I was really enjoying it.

I think I probably need to rest the elbow, but also make sure I do some gentle exercise to retain the strength in the muscles.

A quick Google suggests that it could easily lasts for several months and that anti-inflammatory may reduce the pain, but do not help healing.