I have always, since the age of 12 when I started having them, had about 5 weeks between periods. Often longer, never less than a month and I can count on one hand how often it's been that short a gap. I once went two months without a period as a teen and 5-6 weeks is normal.
I have conceived 3 times and have never had any obvious problems with fertility and was nicely used to my long cycle. Occasional cramps but never two months running and rarely too extreme.
Until recently.... my last three periods have been the 24/10, 24/11, 24/12 and today; 21/1
All of them have been heavy and painful. On Christmas morning I was in agony... felt like labour pains. The two months before I was woken by the cramps before my period had even started and today I developed a severe pain in my side which radiated towards front and back and down my leg and started bleeding an hour later. The pain was still there until the neurofen kicked in and stayed in my side 
So why on earth would my cycle spontaneously shorten and become so much more unpleasant? My youngest is almost 2 and my periods returned at 17 months postpartum and were the usual 5ish weeks apart until October.