When she is on her period, something is probably hurting. The lower back (or lower back and upper buttocks) is common. Breast discomfort is also common.
Learn where her “period pain” is, and how you can rub or massage her to reduce this pain. You will have to really listen to her, and back off at any hint of “too much” as the line between “that helps” and “that makes it worse” can be razor thin. Once you know what to do, don’t wait for her to ask – do it as often as you can.
