Lump in neck?

So on Sat. I noticed this lump, probably the size of a marble or smaller, and is hard and not moveable. I went to the ER on Easter sunday to get it checked out. The ER Dr. said not to worry and he does not believe it is cancer. He drew bloodwork and everything came back normal. He advised that I see my dr within seven days. I will be going to the dr on Sat. to get this checked out. The ER dr. said if it does not go away in 7 days I will need to have a biopsy done. Today I am very tired, and feel sick, but do not have a headache or a sore throat. My question is, what could this lump be, and why did it appear out of nowhere in one day. Any suggestions would be greatly appreciated.