I just got on the make sure my streak didn't end, but apparently the streaks reset at 11pm? Why not at midnight? wtf.
Streaks end at midnight in the timezone you were in when you first registered for Duolingo.
You may have moved to a different timezone, or your timezone has changed due to a daylight saving scheme.
Other possibilities are you have not met the target XP of your coach. I set my target to 1 XP to prevent this. You can also check the time that your streak ends with the timer on your home page. Check just after 10 PM and it should say 2 hours to go. If it says 1 hour to go, Duo thinks your in a a different time zone than you actualy are.
In that case I would buy a streak freeze just in case.