3 inch mosquito bite?

Last night I was outside and I felt when I got this bite. It's on the inside of my bicep. A couple minutes afterward, it was already pretty large, between the size of a nickel and quarter. Today it's a little over a 3 inch diameter and mostly spread down my arm, rather than an equal distance around the redder mark in the middle. It doesn't hurt and hasn't opened or anything, and I know I'm not allergic to mosquitoes because I currently have about 8 others on me right now, and they seem normal. I haven't been scratching it, but where it's placed is right in line with the hem of a short sleeve shirt so that could be irritating it. Should I be worried about this?