I have always had (somewhat) regular periods, and although I did have longish cycles they would always be regularly and monthly too. But in around December they began to change. I had one that was really late (19 days late) for no particular reason (I AM NOT PREGNANT).
When I did get it was normal then I had a 38 day gap and got another, unusually light period. Then to my surprise, only 12 days after that one started and ended I got another, unusually heavy period. I brushed that one off as a weird cycle, but then it only got weirder. 20 days after that period in December, I got another one. Since then, I've had 20, 28, and now today 23 day cycles. I'm so confused, it really get's me and they're quite heavy too so I'm concerned that they're happening so often. I don't know...
Irregular periods are often caused by hormonal imbalances, in this respect a condition called polycystic ovaries is a common culprit. Another possible cause is problems with the thyroid gland. I would seek medical advice from the GP, this person should arrange for you to have blood tests done.
It is in your interests to find out exactly why this is happening particularly if you happen not to be in the age range to be menopausal.