When she’s on her period, something is probably hurting. The lower back is common place for pain. Breast discomfort is also common.
Learn where her “period pain” is, and how you can rub or massage her to reduce the pain. You will have to listen carefully 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.
And just in case it doesn’t go without saying, this is something you do because you love her, with no expectation of anything in return.