Lump In The Upper Arm And Sore Shoulder. Lump Grows In Size And Consistency. Should I Be Concerned?

I have noticed a lump that fluctuates, somewhat in size and consistency. Right now it is located in my upper right arm ...almost on my shoulder. It is almost golf ball size and very firm. It isnt very painful but can become sore if manipulated frequently. I have noticed that it started out about half this size and firmness and was somewhat elongated but seemed to be dissipating after a week or so. The last 1-2 weeks it has grown in size and consistency and is becoming visibly apparent. It is now about the size and consistency of a golf ball and my shoulder is a bit sore, especially when flexing. Please advise if I should be concerned. I plan to see my PCP next week but my wife is very worried and is questioning whether we should go to our local ER. Thank you