Don't know, but I just want to rant (and sympathise, of course! ) about the same thing... I thought last month's 20 day cycle was due to travelling/jet lag messing up my body clock, but it's arrived after 21 this time, too.
No it shouldn't. My mom had a change in period duration too when she was about 40. She always tell me as if to warn me. ugh. It happens to me too (it can come on the spot or earlier). I feel like I know when it's coming earlier and a few times I've had to stop it coming because of family holidays or weddings. So I went online and found the online clinic which, after a free consultation, prescribed me with norethisterone. I use it now and then and to be honest it's very convenient when I really need to stop it. Don't use it every month though. Best of luck with this!