"Slag" was any inexplicable, sudden death. SOMETIMES it was apoplexy (stroke), but the term was a catch-all. (And yes, even very young children - infants - may have a stroke, quite a recent medical discovery.)
If the mother was suspected of having smothered her child, she would have been had up before a court of law, regardless of whether it had been by intent ("uppsåt") or through negligence/carelessness ("vårdslöshet") or mishap ("våda"), and that would most certainly have been noted in the records.
I believe nearly all cases where it has been actually proved that someone has involontarily smothered a child by rolling over it in their sleep, that person has been under the influence of alcohol or a drug.