Nope, I started it from the game itself not the main menuHmm. Not sure if it'll be fun since you're at level cap.
I think you got the boost because if you start a DLC from the main menu (as opposed from within the game via the story event that kicks off the DLC), you gain a big boost so as to equip you for the DLC. SO in a non-NG+ game you'd start at lv30 or some such.
NoneDid you install any mods?
Last I played was before Blood and Wine came outSounds like the exp boost bug caused by patch... 1.21 I think it was. I remember I killed like one wolf and was boosted up ridiculously many levels. There are threads about it buried somewhere on the forum, and all around the internet.
If you last played years ago it's probably that bug.
I'm not looking a gift horse in the mouthThat's probably it then. Additionally, the level cap was upped from 70 to 100 with the expansion, which would also fit.