Why Has My Menstrual Cycle Become Shorter?

I am 43 years old and my period cycle was always 4 weeks with a 5 day bleed and my periods have always been reqular since I first started, I do not have children and have not taken any birth control pills for about 15 years so why has my cycle changed to 3 weeks with a 5 day bleed. Why has it become shorter? Thank you