I'm not sure why but for some reason my fluency level was rising very quickly when it shouldn't have, it went up a good 6% after just a couple lessons. It seems to have stopped but I was wondering if anyone else had a similar problem?
Duolingo's fluency levels is not a good indicator of how fluent you actually are in a language. I have heard Duolingo's fluency levels are dependent upon how often you get answers right versus getting them wrong. Though it is nice to see a high fluency level, don't worry about that percentage and keep working.
i know, it's supper annoying. sometimes it goes up during the practice rounds, when you aren't learning any new words. and i don't think it's very accurate, as for example, in my german course, there's a tab with words i've mustered so far, and thanks to german grammar, pretty much all words repeat several times, so the number is way over blown. right now it says i know over 900 words of german, when the number is actually closer to 300. however, there is a logic to it. not all words are created equal, like "good" and "bad" and "day" are very common. "presumptuous" is not as common, but it's not exactly rare word either. "petrichor" on the other hand, is extremely rare word, and knowing it, is not gonna help you communicate a lot if you find yourself in the streets of london. so, there's this graph, that shows you how, learning only 200 of the most common words, will get you at the 50% of fluency. 50% is not as much as it may sound, it literally means you'd be understanding every other word native speaker tells you. but it's not gonna be every other word of every sentence, you'd be able to understand basic sentences, and you'd get the gist of more obscure ones, and you won't be able to make anything out if you went to a university lecture on nuclear physics.
the relation unfortunately isn't linear, so learning next 200 words, won't even get you to 60% fluency. as words get less common and more specific, you'd have to learn more of them, to go an extra percent up, so, to reach 80% fluency, you need to learn total of 3000 words. however, with 80% fluency, you can pretty much lead a decent conversation with reasonable person who's not using words such as petrichor.
then again, even when it comes to your native tongue, you don't have a 100% fluency, so there's no need to fret about it. the more you learn, the more you understand, the more you can convey, and that is all you can hope for.
you can see the diagram of this phenomena on this page http://how-to-learn-any-language.com/e/guide/vocabulary-learning/how-many-words.html
that said, i don't think the % shown on this page is very accurate, cause the words are being miscounted, different forms verbs take after certain pronouns, or cases nouns take, shouldn't be counted as separate words, but as single occurrence of the said word to create a clearer picture. but that's the basic idea behind the % so it's not broken when it grows fast, it just means you are just starting.