It could be due to a few reasons. Most of the times, it just lasts for a few days. But if prolonged, do seek a PD's or lactation consultant's advice
- does she have thrush or any mouth infection?
- is it just one breast or both?
- tightness in neck muscle
- disruption in routine?
You can read this article for more info. Hope this helps! Hang in there mummy!
http://www.babycentre.co.uk/a8490/when-your-baby-wont-breastfeed