Got the solution of previous bug. You have to sleep to level up. When I slept and woke up I got a level up and then I slept again and another level up. My two level ups were pending because of that. Also you have to sleep on a rented bed in your manor.
Actually, the game's tutorial lets you know that you need to sleep to level up. The first time you hit max exp for a level , a pop-up should have informed you regarding the "Sleep to level up" feature. Surprised you missed that
Btw, hope you know that Quest rewards are scaled to your level. If you are at level 10 , and you finish a quest , you get a level 10 version of the reward (for example a dagger with 5pts/sec poison damage). If you were at level 30 when you did the quest , then the dagger would have 30 pts/sec poison damage.
Fortunately, there's a mod that fixes it. It always gives you the maxed out version of the reward regardless of your current level. Highly recommend you use this.