This is adorable.
Tell her you love her and want to support her and then ask her what she wants during her time of the month… when she’s off her period. Like ideally a few days to a week after (that tends to be when we are in the best mood FYI).
Thank her for being honest and let her know she has the right to change her mind when she’s on her period and you will do your best to accommodate. There’s no silver bullet answer, no two women are the same and no two periods are the same! Seriously dude, it changes every month. For me the experience of a period is better or worse depending on my stress level at the time and how well I’ve treated my body that month with food, sleep, exercise, etc. Be understanding, actually listen and be flexible.
Generally I think allowing someone to change their mind (even about small stuff like what to watch on TV or what to eat for dinner) and even supporting them in doing so is one of the most reassuring and calming things out there when you’re all emotional. Basically: don’t force decisions that are unnecessary and let her live “in the grey zone” for a bit.