My cousin is allergic to bug bites, and so he swells up like that too. You should have seen his eye one time when he was three. He got a bite by his eye, so all the face around it swelled up and turned black, and you couldn't even see his eye By brother is also somewhat allergic.

I'd put ice on it and take some allergy meds. But I'd also go to the doctor. They might decide that something is wrong, or possibly it needs to be lanced.

good luck!