What are the possible causes on ongoing stinging sensation in one leg?

My lower leg, directly above the heel, is sending a stinging sensation every thirty seconds.  What might be causing this?  This began about an hour ago....and has never happened before.  It's a burning sensation, painful for a few seconds and then subsides.  Any ideas?  I got some temporary relief with Benadryl gel....but it's still happeing but less intense. 

Thank you for any ideas about what may be causing this.