My breasts do get sensitive before my TOM, but I haven't noticed the low carb making it any worse. I've heard evening primrose oil can help with the symptoms of PMS, including but not limited to breast tenderness. My friend uses the oil, and swears by it. Vitamin B-6 my also help, but take no more than 200mg daily and divide the doses into 50mg at a time- it helps produce progesterone which balances out the excess estrogen. These are 2 natural treatments you could try to help you with your battle against PMS. It's worth a try anyhow. Hope it helps. Good luck.
God Bless,
Amy-Lynne