I wouldn't add anything to your diet. Irritable bowel issues can be problematic when hormones raise and fall. If U add something 'unknown' you won't know what is/not working.
10 mins. B4 a bleed started I would have to dash to the loo, even if a period wasn't due. No argument !! make way !! When younger I also didn't poo in the 5 days prior to a period ...... so definitely hormones!
Make a diary of what you eat and drink for 5 days/nights. Everything. Loose bowels can be caused by anxiety, a bug, too much liquid and not enough roughage. Eating little and often with lots of slow release foods, i.e. porridge, bananas, LIVE yoghurt for 2 days then introduce veg. and fruit.
Is the poo yellow/watery/offensive? If so you need to talk to your GP. When I had those symptoms I was diagnosed with slow transit, that was way back in 1997, appendix already removed by then. Advised more roughage but my diet was already really good despite eating disorder.