Rear hub loose after several days in the car
I suspect that somehow you've loosened the locknuts while disassembling, and possibly days of vibration have allowed a the nuts to migrate along the thread.  Totally a guess.

The fix is simply to readjust the bearings correctly, and then make sure the locknut is cinched down onto the cone to prevent movement.

If your wheelset has sealed or cartridge bearings then this supposition goes out the window, and something else is wrong.