Is this a mosquito or a spider bite?? Help!?

I have this really big thing on my leg just below my knee, and I don't know what it is.... It's really, really itchy and it's raised up at least a millimetre and it's the size of a toonie ( 2 dollar coin). There are no bite marks or holes on it, and it hurts a bit. I read that it could be Cellulitis, but i think that happens to the whole leg. Is it just a really bad mosquito bite??

Thanks for you help! :)