This is probably not very helpful (and it is most definitely not based on anything medical) but do you know what, after being on MN for nigh on a year I have come to the conclusion that sometimes our cycles just change for a bit for no apparent reason. I too have had weird changes to my cycles over the years (shorter, longer, heavier, more painful, you name it) and I have lost count of the number of threads I've seen from mnetters with similar problems (and that's just in one year). I've been to my GP about it more than once but they can never explain it. Since I am otherwise healthy I've decided to stop trying to understand it and assume it is just part of growing up old.
But do check with your GP obviously. We are all different and our bodies will do different things for different reasons.