I had a minor op on the back of my head about 2 weeks ago

[Saltire]

and about 5cm below where the scar is there is a very sore, hard lump - it's sore when I touch it, and it has moved further down IYSWIM since I first becamse aware of it last week.
DH thinks it probably where the local anasthetic was injected (the op involved cutting the skin with a scalpel), but I admit to being a bit worried by it now

This would be about the right time for a reactive lymph node - gland processing inflammtory material - to be enlarged. It may be enlarged because of infection in the wound and the best thing to do would be to get your GP to have a look at it - ask for an urgent appointment which will get you seen on the day. It won't take long to eyeball it. It is not all likely to be anything serious though.