The same thing happened to me, only since I've always been somewhat irregular, now I play the "when is it coming" guessing game every 3-4 weeks instead of every 4-5 weeks. I went in to my doc, just to be sure, since menstrual changes can also signal nasty stuff like ovarian cancer, and my family has an unfortunate history with such things. She sent me for an ultrasound, but in the end the diagnosis was "40 year old ovaries." It's not menopause yet, or even perimenopause, but for many women as we hit around 40, our systems wind up for a last shot at reproduction, and our periods may also become more of a roller coaster ride because our hormone levels spike and drop instead of rising and falling gradually through the month.
So, you may want to consult your health care provider to make sure nothing else is going on, especially if there is a cancer history in your family. But most likely, it's because you, too, are the happy owner of 40 year old ovaries.