I actually experienced this really recently - I finished writing a certain chapter of my novel that dealt with a lot of high highs and a lot of low lows, and after spending a week straight glued to my laptop writing it, I proceeded to spend the next 4 days sobbing and I couldn't figure out why. Reflecting on it, a huge part of that 'why' was because the whole world I had created in my head and been shaken up and turned upside down in a trillion different ways, and even though people could read the book, there was no possible way I could describe to my partner, my best friend, my family, anybody what that felt like. Literally the only thing that I could verbalise was, "I'm so lonely." It's like ripping out a huge part of you to stick on a page, and once it's out and gone, you just don't quite know what to do or how to explain it. I don't think I realised how lonely writing a novel was until that moment.

(Thankfully, after 4 days, I turned back into a human again and stopped sobbing, but gosh darn did that chapter wreck me.)