I am doing the Dawnguard questline and I was getting the last elder scroll from septimus sigmus.
I told her to wait while I was doing stuff with the lexicon as she kept getting in my way. She did however follow me out when we were finished we then went to the dawnguard to talk to the moth priest and we now have to find a moth priests knife.
I went to Riverwood to feed and I'm not sure if she went there with me or not but I have not been able to find her at all throughout the rest of my game play. Nothing on google has helped me it's like she has just vanished from the game even though I still need her for quests.