As a language learner I have similar experience with yours.
One day I could speak EXTREMELY fluently, but the next day I couldn't say anything and nobody could understand what I am talking about. It's not only your problem.
And depends on people, it could differ.
I am quite struggling with talking to old gentlemen because their tones are so low and pronunciations are not so much clear.
What we should do is just keep practising. :) Don't lose your hope!