I'm about 50+ hours in and Lydia died after about 10. I used her a lot in the beginning as bait so that I could get my ranged attack skills up and she got swarmed by a bunch of Necromancers. Now out of no where she's alive again and hanging out in the Dragonsreach (I never bought the Whiterun house).

So... what the hell?

as you said necromancers...(shes a spy)

It's the necromancers.

I tend to burn through followers and view them as disposable. I'm not a jerk or anything, it's just not really worth that much effort to keep them alive. They carry stuff and are a good decoy. This is the first time I've ever even heard of this happening with a follower though...