Suggest Remedy For Red Splotches On Thigh

Hi .Ive got a big circle on my thigh .I think I've been bitten by something because I can see a very tiny bite mark and it started out as a bite and turned into a big red splotch on my thigh .It wasn't bad yesterday but over night it got bigger and spread about 3 centimetres bigger .Its very warm and the skin is quite hard where the bite is .What is it ? and I'm getting worried .