Why did I have to start at -10 XP for the day and one lesson only got me up to 0/20 XP for the day?
It's just a glitch.
Happened to me too.
Actually I had a laugh because I made so many mistakes in the first lesson that for a few seconds, when the counter went from -10 to 0, I couldn't believe that the server at Duolingo just literally told me "you know, you really have to repeat that lesson".
So I actually did it again. No mistakes this time :D
I just had to buy a new streak freeze, my first in two years, despite having done seven lessons yesterday. Something is very wrong.