It's strange - but he does say they are only SLIGHT changes that MAY be due to a toxin - it sounds to me as though it could have been entirely natural causes - just a slightly shortened lifestyle due to her earlier poor condition. After all, many humans don't make it to their full three-score and ten for no obvious cause.
We do know that the span she got was in the height of comfort.
“You lived what anybody gets, Bernie. You got a lifetime. No more. No less.”
― Neil Gaiman, The Sandman, Vol. 7: Brief Lives