I actually got this on a ride recently, in shoes that I've ridden for thousands of miles with no issues at all.
Turns out my socks had snuck into the dryer and shrunk a little, and were too tight. I stopped mid-ride, took 'em off, stretched 'em out as best as I could, put 'em back on, and the rest of the ride was fine.
So short version, before anything else, try different socks.
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