Why did my xp drop by about 600xp?

Question

About three days ago, I reached level fifteen.
Today (three days later) I discovered that my xp had dropped to 14764.....
I didn't lose any recent challenges. ..
..so what could have happened?

Answer 1

Did you reset your progress of some courses ?
Because if you do so all your xp from that courses will also drop.

Answer 2

How did you know that?....
...thats correct though