If it's a soft lump, and the bone is still intact under the lump, then it should go away.

It sounds like it's either a hematoma (old blood as in a bruise), or tissue flooded with fluid.

Any swelling after injury (accidental or planned... as in surgery), is normal and is the result of a natural inflammatory process.

When damage occurs to the small blood vessels in the body, for example when they're cut during surgery, leakage of a thin yellowish serum occurs into the surrounding tissue. This is the fluid you've probably seen when you've had a blister at some stage.

If the swelling has not started to subside after (say) 3 months, I suggest that you contact your surgeon.

We do have a Traumatic Brain Injury Forum here, and maybe you might get some information there.
Here's the link : http://neurotalk.psychcentral.com/forum92.html