This happened to me once too, but it was on the morning after I had allowed Astarion to sample my blood and then failed to persuade him to stop. I just assumed that after I passed out, his bloodlust drove him to drain Shadowheart next in order to sate his compulsion?

Either way, she was dead, kaput, gone for good in that playthrough for me as well, no revival possible.

“This year the utopian candy shell has melted away to expose a hard center of bizarre reality.”