How To Treat Pain In Left Leg And Thigh Due To Cellulitis Inspite Of Taking Keflex?

ive got cellulitus on my left leg, the dr gave me keflex for the infection. i have pain in my upper thigh on the left leg, i have never felt this pain before with the cellulitus when it has occured. i went to the emergency room yesterday and they checked for a blood clot, said there wasnt one, told me to take the keflex and the pain would go away. i have been taking the prescription and the pain is getting worse. what could this be or be from?