I have this red rash on the underside of my arm and it feels very warm ....?

About two days ago, I saw a mosquito bite on the underside of my arm, about halfways, so I just forgot about it. The next day it had grown like 100% in size and it's like this reddish-pinkish color and when you touch it it's very warm. i don't know what it is and I'm freaked out. I don't have shingles or allergies since it would have come up earlier. Any suggestion on what it could be? I'm only 16 and I noticed the bite, which I think it is, while playing tennis in the tennis courts. Please help!