Yes you can. This from the Monistat website: "Q. Can I use MONISTAT® while having my period?
A. Yes, MONISTAT® can be used during your menstrual period. In fact, many women get vaginal yeast infections just before their period because of hormone changes. Using MONISTAT® during your period will not affect how well the product works. If you have started treatment and your period occurs, you should complete the full course of treatment. Do not use tampons while using MONISTAT® because tampons may remove some of the drug from the vagina. Use deodorant-free sanitary napkins or pads instead, and change them often."
Using monistat while you are on your period, is not the best idea. This is mainly because it will all just come out before it has a chance to work. Waiting until your period is over, is ideal. Feel free to consult your doctor for confirmation.