As wounds heal, there will be an inflammatory response and all wounds will swell slightly.
The scalp is a very vascular tissue and hence it heals very fast. It also means that the commonest cause of a swelling in a scalp wound is a blood clot.
Other rare causes include infection with collection of pus and the most fear complication of is the accumulation of Cerebro Spinal Fluid (CSF). The main danger here is the risk of infection in this collection of CSF and may eventually lead to meningitis.