My five year old is suffering from chicken pox at the moment its almost 5,6 days but today I felt lump behind his head I'm bit worried is everything all right or should I go to my dr?

Has he hit his head? Did you look at the lump?

Good pediatricians always say, when in doubt, come in.
Go see his doctor, let him check it out and see if it's a chicken pox or something else.

Good luck.