I also discovered HOW my chronic constipation started: when I was travelling to the other end of the world, my aunt suggested a prescription medication that prevents you from having diarhhea when you're abroad. I can't remember the name right now - but it caused major constipation for me. After that, I also took a herbal medication that was supposed to protect the stomach from toxins but it had a very constipating effect.
I think the constipation stayed with me for so, so long, and even now my BMs will be totally imbalanced if I ever have any dairy. I used to be able to have 2 or 3 BMs a day without even worrying about my diet, but I think it has taken (or is still taking) ages for all that weird medicine to wash out of my system!