There is a limit to how much XP you can earn in a single 24-hour period, which starts at midnight GMT. XP and PP are earned on ticks. There are a specific amount of ticks in a 24-hour period and you can earn XP for a certain amount of them, and they do not need to be earned sequentially (you can earn 3 ticks now, 4 ticks later, etc.)
Keep in mind you will still earn PP even when you reach your XP limit.
We have tuned the XP system to align with when the features that are unlocked at higher levels will be finished and available, including new avatar sets, etc.