Incontinence pads and menstrual period pads are two different types of product. Each of them can handle a different type of flow, which means that they cannot be used interchangeably that well. The slow blood trickle is handled better by menstrual period pads, while the heavier flow of urine is dealt with better by incontinence pads.
Any doctor can tell you that you should only use such sanitary products for the purpose they were created. Some incontinence pads can be used as double protection when you deal with more substantial periods, and you need an extra layer. They can be placed on the bed to protect your sheets.