Round raised bump on dogs leg

Question
My dog has a round, hard, raised bump on his hind leg, about the diameter of a dime. It is not red, and it doesn't seem to be painful or itchy. There is hair over it, so I can't see the skin very well, but it seems normal. He didn't have the bump yesterday morning.

He has had problems with allergies for months, but we control it successfully with Temaril-P (1 pill in the morning), Benadryl (25mg at night), Douxo Calm shampoo/spray, and Omega 3. He eats a hypoallergenic lamb and rice diet from California Natural. He is almost 2 years old.

We went for a hike in the woods yesterday, and there were plenty of mosquitos and other bugs. He also went swimming on a lake with other dogs. Could this be as simple as a bite allergy? What else could it be? Should I rush him to the vet?

Thanks!

Answer
IF this just cam up, it may be an immediate hypersensitivity reaction to something in the woods.  Your Temaril-P and Benadryl may help reduce the size fast, so I would just watch it for a few days.  If it doesn't go away or if it gets larger, let your vet look at it.