Go into your app drawer find the Clock app. Press and hold on icon until a small menu pops up. Click App info then go to Storage. Clear the cache and the data and see if it still happens after that.
Mine started this ever since I installed the new update. I think it's at the same time as the installation (9:00 am). The most annoying thing is there is no way to silence or dismiss it. I have to completely restart my phone every time.