At 40, it's time for your hormones to go all wonky on you! There is such a thing as early menopause, if your periods remain irregular I'd see a doctor as a doctor can check your hormone levels and determine what's going on. Good luck!
I am a 40 year old and usually have regular periods but this time my periods returned just after 12 days. How is it so? I had sex on 4th day once. I exercise regularly and have healthy lifestyle.