I have a bump on the back of my neck. And it's draining pus. What could that b?

Pustule/abcess. Occasionally the oil glands or hair follicles are invaded by skin germs. Various forms of staph or strep are the most common agents. Staph more commonly stays local while strep causes redness & extends under the skin surface. A warm compress over the area increases blood flow and allows the body to heal itself sometimes, if it persists, your dr may need to open & drain it & rx meds.