Tootie may have known that something was amiss w/her kittens. When I rescued Pidge, her 3 kittens were born prematurely and she kept lying on top of them. She wasn't trying to nurse; she was trying to smother them. I had to remove them from her but they all died anyhow. But if that isn't the reason for Tootie's behavior, it still isn't something that you could've second guessed or that you can blame yourself for so try not to feel to badly about it. I know it's difficult, especially when you came upon such a scene as that but it wasn't something that you could've foreseen or you wouldn't have let them be together.