What Causes Swelling And Tenderness In The Head?

In the last two days, the left side of the back of my head has become very tender to the touch and a bit swollen and/or lumpy. I have not injured it in any way that I can think of to cause this. I can feel the bump or swelling when I tilt my head back as it presses against the top of my back. My scalp shows no signs of any irritation or rash. I do not have a headache but the afftected area is sore. I am in good health otherwise with no major illness and am not taking any prescriptions other than a Vitamin D. Any suggestions? If it doesn t clear up after tonight, I will see medical attention as this is day 3 of this.