Could your shoes be a little soft for cycling shoes and allowing your feet to collapse a little around the cleats? I had this happen the one time that I tried spinning in my Keen Commuters with cleats and got different types of foot-related problems in that class. I do not especially remember any numbing, but there may have been, there was a lot going on that session.

The Keen Commuters no longer have cleats and are my platform riding shoes once again...