I Woke Up This Morning With Only My Left Foot

I woke up this morning with only my left foot having a red, rashy, hot to the touch, and full of small red tiny blisters. It's mostly around the upper ankle area. What could THIS be? Do I have a reason to worry? Should I go to the doctor ? I keep on taking 2 Benadryls every six hours because I do believe that I have an allergic reaction to WHAT ever has bitten me. What do you think this could be?