5 year old woke up with a swollen hand, has a small pimple and it has doubled in size?

Usually bug bite: This usually represents an arthus reaction (allergic) to mouth material injected by a bug bite. The body causes local capillaries to release fluid to slow down entry of any toxic materials into the system. It usually lasts a few days until the system removes the offending material. Rarely they get infected. If it spooks you have it seen. Benedryl can slow the swelling some.