What causes an itchy swollen bruise without any injury?

Insect bites can do this, and the itching makes me suspect this even moreso. So can bumps you don't notice. See if you can examine near the center of the bruise and find the actual bite. Treat bruises with an ice pack, 10 minutes on, 10 minutes off, for half and hour. Repeat as necessary. While there are a ton of other reasons for this sort of thing, both minor and grave, I'd suggest looking for the easy ones first.