Be very glad you got tired of storing XP. Evony has quite a few bugs when it comes to VERY large numbers. More than a few people have gotten too much experience and their hero's XP glitched and rolled over. The XP passed the max threshold that the Evony program was made to calculate, and suddenly the player's hero had NEGATIVE 2.5 trillion experience.

You probably dodged a bullet.
Good work on that hero. I'm glad to see you did this with a helm of mars POL hero, rather than a "good enough" hero from the inn. +rep for that.