What could be the cause of a head swell? did hair dye before this happen. could this be the cause?

Your questions is not clear. Do you feel that your head is swollen? Did you measure the circumference of the head and it has increased? If you measured it, how much did it increase? Do you have any other symptoms? If the issue is still a concern, it would be prudent to see a doctor as it would require an in-person evaluation. Wish you good health!

Am allergy to the dye is always possible and appears likely in above circumstances. Other conditions cannot be ruled out. See your doc in case swelling does not resolve in the next few days.