During menstruation, there is a chance you can be fertile. Apparently, the body starts preparing for ovulation already when bleeding occurs. Women with short cycles can be fertile when they are bleeding and during the days following bleeding.
So, the assumption that during menstruation or the days right after you are not fertile, is wrong.
By knowing your hormonal patterns and the characteristics of your cycles, together with observing your fertility signs, you could know when your fertile days are actually happening.