Lower Trap & Shoulder Pain getting worse over time caused by improper form during pull up. Thoughts/advice?

Last week I started practicing pull ups. I cannot do even one so I started with negative pull ups - they are where you have to jump up onto the bar and slowly lower yourself down to train the muscle. But during one of my reps I didn't jump confidently and ended up pulling on the bar and falling at the same time. I don't know if that makes sense but I felt a twinging and shaking of the muscle in my shoulder and upper back on the left side. There was some pain but it didn't hurt that badly. There was no pain for days afterwards.

However, as days went by it slowly crepted up on me. After taking a long walk I would feel a numb aching feeling in my shoulder and lower trap on my back. Which went away after sitting down. Then I could feel it when I just stood still. And now I feel it constantly.

I am wondering how serious this is? I can't really afford to see a physio at this time so I am wondering if there is a way to self diagnose and if there are any exercises I can do? Thanks