What Causes Vaginal Odor During Periods?

I am a relatively fit and healthy 29 yr old female. This latest menstrual cycle has not been normal at all. It is my second cycle since coming off the contraceptive pill, the first cycle was fine but this one is odd smelling - almost sweetish and rotten at the same time... I feel fine and there is no itching or discomfort in the area but the smell is definately not normal. What do you suggest?