Numb skin post-fast (or at any other time) can be caused by a lack of peripheral blood circulation, and where the nerve-endings are not receiving the blood supply that they should.
Sometimes more urgent work is being carried out elsewhere deep within the body, which requires more blood to accomplish healing, just as more blood is required after a hefty meal and the blood is drawn away from peripheral areas.
There can be a number of causes, not least of which can be stress or anxiety as well.
My guess is that this is only a temporary phenomenon and nothing to be overly concerned about.