This week, I've already experienced this and @kaelci mentioned it too - what Andrew Weir says (thanks for the link @mariannewest!):

"Sometimes, when you’re writing, things come together easily and you can crank out 2,000 words in an afternoon. But other times, it’s torture just to crap out 300 words. In those rough patches, here’s something to keep yourself going: When you read the pages later, you won’t be able to tell which ones you wrote with good flow and which ones were hard. You’re creating the same quality of work in both cases. You might not believe me, but the next time it happens to you, check the results later. You’ll see for yourself. So when you’re having a rough patch, it helps to remember that you’re making progress toward a goal. The words you’re putting down aren’t wasted."