Yes, i have to absolutely agree with MrBumble...
This has happened to me before too. The key thing is not to worry about it. That will make it worse, as you will begin to doubt yourself. Let it rest for a few days (as said by MrBumble), concentrate on something else you are learning atm and come back when you feel you're ready. (For me that moment has arrived when i stop feeling bad about my poor progress. But that just me).