If you have already closed all the Oblivion gates then you cannot give her the bloodgrass. It's some kind of lag which happened to me also. So I got the addon for the Vampire Lair and in there you can find some sort of "bathtub" which will take away your vampirism. But first you must add the weird salt rocks to the water which are located on the walls in the same room. Good Luck
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where can i find that