yes ! It happens to me all the time , not only on understanding a new language, on other occasions too. like dating with a girl, you understand her well last hour, you made her can't stop laughing with you , she know your sense of humor,and all of a sudden, she look at you as a retard, everything you said are impropriety, all your jokes are silly and offensive. and it is the same girl with same me, I don't know what just happened, in China, we call this kind of situation as :"got a cutting-out on brain."

but I guess what I have experienced are not the same thing with you...

Recession for a while to some measure is often seen while you are reaching for something you want. Just relax for a while and go away for a while to clean your head. No worries, you will make an advance again before long.

This happens to me sometimes, too.  Some days when I read Spanish or hear Spanish-speakers talk, I understand them perfectly.  Other days, I feel like I went back to 5th grade Spanish again!  Sometimes it even happens all of a sudden.  The other day I was reading the Instituto Cervantes website to learn about the online classes they offer, when all of a sudden I forgot how to read Spanish!  The website which I had been understanding perfectly was suddently gibberish.  It was really frustrating!

It happens all the time and it's so frustrating. When that happens I need to take a moment and try after a while.  

Fluctulation is inevitable for a English learner to master English. There is a Chinese old saying I am glad to share with you: 3 steps forward means 1 step backward. 

As often as not, fluctuation occured during the course of my English learning which often disturbed me. My way is to keep moving and revise the mistake I made no matter what. My faith is as long as you can keep on, you can keep improving.