I took Lydia on a quest three guys were beating us so I changed from daggers to my sword and fire by the time I managed to pull them out and kill one of the three guys Lydia had died, she was lying on the floor and I could search her. I went got a new companion and went on some quests and went to Breezehome to gather some stuff and went up stairs and Lydia was just standing there. She was dead and I saw it. What going on? Is she immortal?