I had this happen on a bike where I'd removed/installed the wheels so many times that the inside of the drop out was worn smooth. I roughed it up with a dremmel (GOD I love that tool!) and it was ok after that.

Check your QR. Is it the same one you've been using? I have also had some QRs (from one particular manufacturer...cough, cough, ALEX, cough...) that felt tight to close but slipped during hard efforts.

Heal up quick.