I'd wager it's your phone going in to ultra power conservation mode because of low battery. My phone does this when it gets to 5%, though, I think my previous was 15%.
You can mimic this by going into Settings->Battery and turn on Battery Saver to verify this is what you're seeing (my test guesses that's what you're seeing).
Battery Saver turns off certain features to help preserve the battery. I don't recommend keeping it on -- just when the battery gets low.