I have a bug bite that has what looks like a bruise around it?

A couple days ago i got a bug bite on the right side of my right knee, at first i thought it was a mosquito bite but now it has a bruise looking discoloration all around it. the bruise is almost one inch in diameter and is a red-purple color. should i be worried? i have never had a reaction like this to a mosquito bite, i know this for sure because i get them all the time. what could it be?