Borderlands – Accidentally clicking DLC when low level

As I note in this related answer, a mission, rewards and mission enemies scale to the level when the mission was taken. If you take a scaled mission at level 45 and level to level 50, unfortunately the mission and its rewards will still be level 45. This is why it's important to wait until you're maxed out in Playthrough 2 before taking on many sidequests.

TL;DR: On Normal, Scarlett's and Torgue's campaigns are supposed to be taken alongside the main quest, at any point since level 15. Hammerlock's and Tina's - after the main quest. On TVHM and UVHM - any campaign at any point.

Campaign DLCs scale to player's level when you first visit them, but with restrictions in Normal and TVHM. These are:

• Normal:
  • Captain Scarlett and her Pirate's Booty - levels 15 to 30.
  • Mr. Torgue's Campaign of Carnage - levels 15 to 30.
  • Sir Hammerlock's Big Game Hunt - levels 30 to 35.
  • Tiny Tina's Assault on Dragon Keep - levels 30 to 35.
• True Vault Hunter Mode: all DLC scales up to level 50.

Upper bounds is what I observe when returning my level 50 from TVHM. Lower bounds are what I see when jumping there with a level 8 character.

Big Game Hunt and Dragon Keep are the ones definitely designed to be taken after the main quest - other two are actually a breeze if you are level 33-34 after the main quest, and especially when you complete them at 36-38.

Side note: Mr. Torgue's Campaign of Carnage is NOT similar to Mad Moxxi's Underdome Riot - it is not a ladder of lots of rounds on a few arenas. It is what Underdome should have been - a full-sized quest line built around a tournament and several arena (and non-arena) battles (and non-battles).