This happended to me about 2 years ago too. I was stung by something, not sure what. I kind of forgot about it and within 4 days of the sting most of my upper thigh was red, swollen and very hot to touch. I ended up in the ER on IV antibiotics and they were talking about admitting me to the hospital. Luckily I was not admited. The dx was cellulitus. Keep an eye on it and go to the Dr would be my recommendation.