I had this same condition for a few years; it made me re-think my whole window cleaning plan.
I began to sub contract out directly as a result of tennis elbow. My fingers began to tingle and my forearm ached. I alleviated some of this by using my left arm more but it was this condition that told me something had to change. This is the main reason I began to sub out.
12 years later I manage 100%. I have 7 subcontractors that do the labor. I cleaned for 15 years myself.
There are forearm exercises you can do that help. I would suggest research. Cortizone shots are temporary; rest is the best thing you can do if possible.
The condition is called "tendonittis". Inflamation of the tendon in the foremarm due to repeated stress. You can wear a support that keeps the tendon from moving excessively. And take an anti-inflamitory drug such as aspirin or naproxin sodium (advil).
It takes a week or so to make the inflamed tendon feel better but it does help. Exercises, medication, a support to keep the tendon from moving, and rest. Re-evaluate the method that is causing the strain and change it.
Hope this helps;
Hugs,
Texas Girl