New max level cap that came with the new DLC, unfortunately you can't start to earn XP and level up over 30 until you finish the story from the main game (this is mainly for new people who haven't got to 30 in the main game yet)
My suggestion would be to leave as many landmarks, collectables and side missions that you can while finishing the story (I believe to progress in the story you need to get each area to at least 50%)
Then once you finish the main game story the Truth Quest will kick in and you can finish everything off giving you a boost to 40 before even starting any of the DLC missions.
If you completed the game 100% before getting the DLC then you will still more than likely hit 40 before you 100% the DLC anyway so no need to worry, no grinding should be necessary.