The amount of playing you're doing is unlikely to be the real cause, because it isn't enough to get a RSI - repetitive stress injury. I know because I have tendonitis in both arms and got it after 2 years of classical guitar at the music degree level (hard stuff), where I played 8 hours a day for 3 months straight at one point. Even then, the injury didn't flare until a year later.

In my case, other things (like heavy piano, doing pushups, etc.) also contributed to the "over-use" injury. You sound nowhere near "over-use".

Consider what else you do with your arms that is arm-intensive. You should see a physical therapist as well for both diagnosis, possible treatment, strengthening exercises, and proper stretches.

Hope you feel better soon.