It isn't unusual. My mare Freyja did this, about 2-3 weeks before she delivered, and again about 3 days before she delivered. When the foal changes position in preparation for birth, it can cause some contractions, discomfort, and colic-like symptoms in the mare.
However, and as ALWAYS, when in doubt, call your vet.
ETA: And believe me, as I through with my last mare's pregnancy, the "normal" symptoms of impending delivery do not always go by the book. My mare showed all the signs of readiness at around 300 days, and didn't deliver until 360 days.
Last edited by Indyhorse; 09-19-2010 at 11:05 PM.