“He told police that she was suffering from post-partum depression, was having trouble sleeping and was on medication.”
Yeah, she told the boyfriend it was post-partum *depression* but it was probably more like post-partum *psychosis* or worse.
depression can turn into psychosis if it is agitated and people are unable to sleep. Medications used for schitzophrenia are also used for other kinds of psychosis in the short term.