Many women suffer from constipation before their period, which is usually due to a rise in progesterone. When levels fall again, at the start of the period, things usually return to normal. For others, it’s diarrhoea that’s the problem, and this is down to a higher level of chemicals called prostaglandins circulating round your body. Prostaglandins help the uterus to contract and unfortunately can have the same effect on your bowels too, causing looser stools – it’s as simple as that!