Try the following in order as needed:
1 - reboot and test to see if the problem persists.
2 - if it does as a test launch Photos with the Option key held down and create a new, test library. Import some photos and test to see if the same problem persists. Does it? (This tells us if the problem is limited to your current library or is more wide spread)
3 - Next, if the problem still persists, log into another user account and see if the problem persists there in Photos. (This tells us if the problem is limited to your user account or is system wide).
4 - Boot into Safe Mode (Use Safe Mode to isoloate issues with your Mac) and test your library from there. See if the problem persists. Reboot normally and test again.
NOTE: Safe Mode boot can take up to 10 minutes as it's doing some system cache cleaning, volume verifying and directory repairing.
