What Causes Large Welt On The Back Of Leg?

I have a large welt on the back of my leg. I'm guessing it's from a mosquito bite, but I'm not sure. It is a little tender but doesn't itch at all. It's oblong but about the size of a half dollar. I have had allergic reactions to different bug bites, but they usually itch. I took an allergy pill as soon as I saw it, but it's a new thing for me. Should I be worried about this?