Ok Todd, hopefully you have already taken her to the doctor to discover it is a swollen gland. My Grandson gets them too. He has no pain, color is normal, and he inevitably has either an ear infection or strep throat. She'll be ok.
She probably bumped herself so long ago that she does not remember. Trauma, even very mild trauma, can cause a cyst to form, and it takes time to form. Do this: feel the bump. Can you push it and feel some resistance? Does it move around, even slightly? It is probably a cyst. If it is close to the neck area and it feels hard to you, it is most likely a lymph node. When my daughter was in early grade school she had hard little nodes at the base of her skull near her neck. I was so worried about them. They were lymph nodes--doing their job. Sometimes they get hard because they are draining lymph--and they can feel quite hard. If you daughter is otherwise healthy and had a normal appetite, I'd just keep and eye on the bump for the next few months. See if it changes in any way. Don't pick at it and poke it. If you think it is a cyst and cannot resist poking it, then at least sterilize the area AND the needle with alcohol. Otherwise, be gentle and try not to bring attention to it too much. You don't want to worry your little girl needlessly. Hope it all works out!
I once found a similar "knot" on the bottom in back of my neck. It turned out to be a sebaceous cyst (covering over a cyst with a normal body tissue that we have) Another reassurance the doctor told me that if it was moveable, it was a good sign. I could finger the outline and was able to slide it around in very short distances. It never got larger. I forgot about it. Years later I realized that it went away. This is just a comment, not a diagnosis is intended.