I was playing a vampire wizard, when for whatever reason my movement speed became nearly 0. I could move but just barely, like the character was hopping on toes. First I thought it was some kind of encumbrance, but taking off armour didn't help at all.

I had to quit to main menu and reload my last save. It seemed to solve the problem, but still it was very annoying.

Is this some kind of bug? Or something I'm not aware of?

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Sorry me yes dumb.. it was probably encumbrance.

Yeah, everything you carry (not just what you're wearing and currently holding) has a weight rating. Your strength affects how much you're slowed down by heavy loads...and unless vampires get more strength than the average human (I don't have the DLC, so I don't know), a vampire wizard is still a wizard, which means pretty low strength.