Guess: Lost weight and/or less water retained, feet less puffy or smaller, shoes fit looser. (http://www.fieggen.com/shoelace/knots.htm
for ways to tie your running shoes that might work better to hold things in place.)
Second guess: Gait change (intentional or un-). You'd know if this were the case.
Third guess: Old socks and/or insoles, need to be replaced.
As an interim solution, have you tried taping your hotspots while you run?