Out of curiosity I googled this. The most interesting post I saw was someone who found she had lactose intolerance, and when she started avoiding lactose, her feet shrank. I also found this: So, your feet could be getting shorter because your arches are getting higher or because you are supinating more than you had in the past. But both of these conditions are fairly rare. The most common reason for a shortened foot length has to do with toe alignment.
If you’re concerned, why not ask a podiatrist?