This resource suggests a calcium supplement around your cycle to help with supply:
For some women, the drop in milk supply and nipple tenderness associated with menstruation becomes more of a challenge. An effective treatment for these symptoms associated with the return of periods is to add a calcium/magnesium supplement to the diet upon ovulation and continue it through the second or third day of a period. The supplement should be 1500 calcium/750 magnesium but can be as low as 500 calcium/250 magnesium (the higher the dosage the more effective and quicker the results). It should be a combination pill. This much calcium should never be taken alone. If your cycles are not regular and you do not know when you ovulate, you can take the supplement the entire month until you begin to see a pattern to your flow. This type of supplement seems to work as it prevents the drop in blood calcium levels which occurs mid-cycle and continues through the second to third day of a period. It is this drop which is associated with the nipple tenderness and drop in milk supply as well as the uterine cramping so often experienced with menstruation. You only need to take one pill a day.