Everyone gets a bit stiff once in a while. We spend a lot of time practicing difficult passages and pieces and that can sometimes create tensions affecting the embouchure and making our playing uncomfortable especially in the high register.
If your embouchure doesn't feel as good as it normally does, you better not insist too much and avoid forcing the notes to come out. It will only make things worst. If you can normally play a piece well, there is no need to fight with it if it doesn't work on that day.
Instead of trying to beat your instrument into submission, just put the hard stuff away and play some very simple, easy and slow pieces. Even beginner pieces. Stop on a few notes and try to make them sound as good as possible without forcing. After about 30 minutes of doing this, you'll feel your embouchure relaxing and feeling right again.
After that, take a break or put the horn away for the day. When you pick it up again, go back to what you normally practice. You should feel much better.
I am associate principal horn of the Singapore Symphony Orchestra and the author of the progressive methods. I'm happy to share my experience as a horn player and teacher with you.