For the second time this year I have spreading sores on my left leg and knee. They start as pimple like bumps, hot to the touch and turn into oozing sores that leave a raw spot, sometimes purple and swollen. It makes my whole knee really hot and they appear one at a time. What could this be?

It sounds like cellulitis, a skin infection that can lead to blood poisoning and even MRSA if it's not treated. Whether it is cellulitis or not, please see a doctor.