Basal body temperature a definite sign of ovulation?

For the past month or so, I have been charting my basal body temperature in order to get to know a little more about my cycles. Is a spike in basal body temperature and a sustained rise until time of menstruation a definite sign of ovulation, or is it possible that it could happen even in an anovulatory cycle? My BBT was actually only "down" for a few days last month, and didn't drop until a week or so after my menstruation.