Leveling Bug?

Hello, I have reecently started playing as a undead warlock with a friend that plays undead rogue, and I cannot understand why we are doing the same quests, playing together for 2 hours and he has become level 16 meanwhile im 8, really can't understand why im getting 300 xp on a quest that he has got 3000 on, that doesn't make sense.

Anyone has a clue on this problem? a bug? anything that I need to fix?

Rightclick on your experience bar, maybe you have set lower exp rate for yourself.