A temporary dip in milk supply for several days around your period is a well-known and common pattern, caused by the drop in estrogen and progesterone right before menstruation. It typically recovers on its own once your period ends, exactly as you are describing.
Next Steps
Keep nursing or pumping as often as usual through these days, since demand is what keeps supply up even when the output feels lower. Staying well hydrated and eating enough through this window also helps.
Health Tips
If the dip starts lasting longer than your period itself, or your baby's weight gain becomes a concern, please see your gynaecologist or a lactation consultant.