They probably figured that she got a little too much painkiller. When something unexpected happens the human mind usually tries to attribute it to something that is already known.
According to the article, her medical records said never been given painkillers (although it could well be she's never asked for painkillers, which I suppose is a different thing all together.)
In the UK at least, most women only use Entonox (a mixture of oxygen and nitrous oxide) for pain relief, and painkillers are only administered during lengthy pregnancy's. I wouldn't be surprised if the doctors/midwives were a little baffled, but just figured she was a hard nut.
This turns out to be very dangerous for the mother. I saw a documentary about a group of women with a similar condition and one almost died because her pelvis broke during delivery but she didn't realize there was a problem until weeks later.