A few questions...do you have regular yearly checkups from a gynecologist? Are you in your 30's or 40's?
Many times, periods can change due to hormonal imbalances; polyps; endometriosis; or being perimenopausal or all of the above!
Don't fret too much, I'm in the same boat at age 50 and can't wait for menopause. I've had two D&C's to remove polyps plus I have an elarged uterus full of fibroids within the wall.
So my advice would be to get an exam and go from there. If you're not bleeding all month long, then chances are it's just one of a number of things that happen during the years we have our periods.