Confused about periods

Nikki
Although I have pcos I get semi regular periods in a cycle of about 32 days not always spot on but a day or two either side. However one month I will get a regular period with bright red blood for about 5 days and then the next month I will have a gunky brown period that isn't very heavy and not at all 'bloody'. It happens every second month!!! Would anyone know why this happens?