The cycle is very important for women. I didn't menstruate while I was travelling in India, but returned immediately when I came home. I had a friend who restored her cycle just by smelling the herbs used for uttara vasti, she hadn't even prepared the decoction yet.
Stress is perhaps too simple of an explanation. I think I talked myself out of menstruating for various reasons. Important to be comfortable with one's femininity at the deepest level.....uttara vasti can help with that.