I have Fallout New Vegas for PC (obviously) with the 4 DLC add-ons that increase your level cap, but after I got to level 29, it's been significantly harder to level up. Before, when I got 1,000 XP, it went around 1/3 of the bar, but now when I get 1k XP, you can barely tell it moves. My XP on the Stats page says something like 20,000/60,000. Why is it so high on level 29?

The higher your level is, the more experience points are needed to reach the next level. It only takes 200 experience points to reach level 2; by the time you hit level 29 you had accumulated 62,300 XPs, and you need 66,700 XPs to reach level 30 (assuming you're not using any mods that alter leveling). That means you need to gain 4,400 more experience points. This is a very common game mechanic with RPGs.

Why are you confused by this? The higher your level, the more XP you need to get to the next level. It's not a static thing where every level requires the same amount of points, it increases each time you level up. That's the way it's supposed to work.

Please reread the OP. I have 20k/60k XP on level 29. 1k XP used to fill up 1/3 of the bar on level 28 or a bit lower but now 1k barely moves it.

If you have 20,000 XPs and you're level 29, then something's glitched in your game. Are you using any mods that alter leveling?

Only the DLC and a mod that stops the Gomorahh glitch.