TMI.. Always have diarrhoea first couple days of period?

So this has been happening since I first got my period. On the very first day I start bleeding/having the cramps telling me I'm going to start bleeding that day, I also have very.. Loose? Stool.. Is that the right word?
If I'm constipated and about to start my period, first day of my period that constipation is gone and everything just comes out. And then the first 1-3 days of my period I just have diarrhoea to very soft mushy stool.
Whats really weird about all this is that I have chronic constipation issues (despite my diet, water intake, exercise, i get constipated often and extremely easily).
So I don't understand why right when my period hits my body decides to have completely oppostite bowel movements to what I'm used to then when periods done I'm back to my normal, sucky constipation.
Alrhough its kind of a good thing this happens to me, its still always kind of worried me, as it doesn't seem normal?