After several visits at the orthopaedic specialist and an MRT scan, I finally found out the reason for my shoulder pain: Slipped disk in my neck. I really didn't expect it was something so severe, but it was. Now I got two injections to the incarcerated nerve under a CT and I'm fine. Yay! I could finally draw again (although it was still a bad idea; the pain came back a little after I finished the coloration).
It may not look like it, but the coloring was so annoying.