Recently all of the XP I have gained while in my clan reset. I have never seen that happen before and have no idea why. Has this happened to anyone else?
The update of clan xp highscores was done recently - this updates everyones clan xp. Unfortunately when this happens, there seem to be a glitch that resets peoples clan xp to zero - you aren't the only one that this has happened to Unfortunately from what I can see, although Jagged are aware of this happening, they don't know what causes it, and the xp is not reinstated.
You could try submitting a bug report to help with their investigations.