Yeah Louise, I get the one at the back of my neck also. Such a pain, but it's a result of neck surgery. :/
Whereabouts is the lump? You do have muscles in your scalp, and just like your back or shoulders, can get 'knots' in them. That would explain the shooting pain when you press it. I get a pretty big lump at the back of my head sometimes due to my neck being out of place, and sometimes smaller lumps near my temples, and it does shoot pain through my head if I press. In fact if I press hard for a really long time, sometimes the knot releases after a while, but I have to do deep breathing to get through that pain which can go for 30-60 seconds and be very sharp and intense.
Would be good to get it checked as it might not be what I'm describing, but thought I'd just let you know a possibility...
Do they know what it is yet?
Just woke up and the lump is so painful...