What can cause lumps on back of your head and neck that just appeared over night and are kinda of tender?
It could be inflamed lymph nodes which are part of you immune
system. Thus, your body may be fighting bacteria and are causing
your lymph nodes to overwork and become larger.
There's a large concentration of lymph nodes on the back of ones
neck, under the armpits, in the groin area, etc.
Please see your primary care physician to have them checked to rule
out any potential problems.