Pain In Muscles, Back And Chest, Get Worse While Breathing Deeply, Lump On Chest

Hello, After a particularly hard workout, I had major muscle pains that went away after a few days. However, there was (is?) a persistent pain in my upper back and chest on the right side that would get worse when I would try to breathe deeply or laugh, or exert any effort. A week has passed and the pain is almost entirely gone but now, there is a lump on my chest where the pain used to be. It doesn t hurt, the lump is a hard mass. No other symptoms. What could it be?