Possibly a cover up for an illegal abortion. You do not say how many children she had given birth to or how long it was after she last gave birth.
She had 7 children, two of whom who died in childhood. The last child she had was born in 1860 7 years before she died.
The Illegal abortion theory was actually one of the first things that came to my mind just because of the vague description of her cause of death, because of the fact that she died at an address who I believe she had no familial relations living at, wasn't her home, wasn't a hospital or maternity home, and because of the mysterious informant who (according to census records) was not a midwife at all but instead a charwoman or a house cleaner of some type.
I'm only hesitant to accept this theory because she was a married woman who had been married for many years, and from what I know abortions in this era were mostly sought out by unmarried women with a lack of means or support.