I have discovered a pea size lump on the back on my 18 months old head it is located behind his ear towards the back of his head but still in the hairline. It doesn't seem to bother him but I am not sue how long it has been there, possiblely today or a couple of days. It is not red at all. What is your advice...

Is it kind of soft and moves around? Does he have a cold or a virus? It could very well be a lymph node fighting off infection.

It is probably a lymph node. I went to the dr. the first time my firstborn had one.