R.B.
my daughter is allergic to mosquitos and gnats. They dont' swell up right away ( even the mosquito bites) but by the next day they get swollen.. they have gotten the size of quarters before they start to go down.
Try giving some benedryl for a day and see how it reacts. It could just be a small allergic reaction. If no better after benedryl and hydroxortizone for a full day then make a call to the Dr.