Best remedy is always -- increasing your pumping sessions, aye..
Excerpt #1 from here
Although women do not ovulate during early lactation, the later stages of prolonged breast-feeding are frequently accompanied by menstruation. The factors influencing the onset of ovulation during breast-feeding form the main subject in the second part of this report. The reason why there is sometimes breast refusal during the menstrual cycle is unclear, but Hartmann and his colleagues (3) in Western Australia have found major, acute changes in milk composition occurring in the mid follicular and mid-luteal phases, though their exact signification awaits clarification.
Excerpt #2 from here
The return of menstruation does not mean the end of breastfeeding. The milk does not sour or "go bad" when you are having a period. The milk is no less nutritious when you are menstruating than when you are not. Some women do notice a temporary drop in milk supply in the days just prior to a period and for a few days into one. This is due to hormonal fluctuations. Once the period begins and hormone levels begin to return to normal, the milk supply will boost back up again. Most babies can compensate well for this temporary drop in supply with more frequent nursing.
Nipple tenderness occurs for some women during ovulation, during the days before a period, or at both times. Some mothers report a feeling of antisiness while nursing at these times, too. As with the drop in supply this is also hormonally influenced and therefore temporary.Some babies may detect a slight change in the taste of the milk just before a period, again, due to hormonal changes. These same babies may nurse less often or less enthusiastically during this time as a result.
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.
The herb, Evening Primrose, is also reported to alleviate nipple soreness brought on during ovulation or before a period. The dosage is one capsule per day.
Most women can breastfeed without a decrease in milk supply when menses returns. If you notice a decrease, try to nurse more often to build up the supply. This is the best way to treat a decrease. If that does not seem to work, you may want to try a calcium/magnesium suppliment (2 to 1 ratio). You must take half the amount of magnesium with the calcium so it will be absorbed. I'm not sure why this works, but it has been proven to increase milk supply after menses has returned (I know from personal experience!) Do not take coral sources of calcium because it can cause allergic reactions in your baby. For more info on dosage, see http://www.kellymom.com/herbal/natural-treatments.html#calcium.
And more #4 from here