What Causes A Soft Lump On Left Side Of Shoulder?

Whow. I didn t realize someone would be available to answer a question. How wonderful. I woke up one morning and my partner (who sees my naked back all the time) said, heh-what s that lump near your left shoulder blade? It is in just the spot where I can t see it or reach it to feel it. In a mirror I can barely see it. It moves around a lot. Is maybe the size of a smallish lemon. Very soft. Mobile and appeared overnight. Weird. Now - I have to say that I recently started to play a bass guitar and I had just practiced and played for about 3 hours the day before without having played in a months time. My neck was sore from that (the bass weights 20 pounds) and I have recently had swollen glands and been feeling under the weather a bit. Flu like maybe. What do you think? Should I be running to have it checked. (I m one of those people without health insurance and can t afford doctor visits.) Thank you for ANY answers or peace of mind you can give me.