What is your age? That could make all the difference in the world. If your 40 -45 ish then it is common. It could be menopausal stuff.

If you are 35 -40 ish then it is not unheard of. Pre- menopausal stuff maybe, or beginnings of a pregnancy that didn't complete. That happens a lot.

25 - 35 ish strange, again pregnancy that didn't complete. Other hormone related stuff. Stress, change in meds.

Hope this gives you an idea of what could maybe be going on.