Try this, it worked for me and the women in the Breastfeeding clinic I went to also suggested it.
before sitting down to breastfeed, apply a moist warm compress to your breasts (a towel wet with hot water works fine, just make sure it won't burn!), or take a warm shower and make sure some of the water directly hits your breasts, this helps the milk flow. AFTER your baby is done nursing, try pumping each breast for a few minutes to get anything your baby left behind. Then after an hour, pump again. Save the milk. if you are worried your baby is not getting enough breast milk, save the milk you express and you can then either use an infants medicine syringe to feed her, or one of those supplemental units that has a tube that tapes to your breast. if you give your baby a bottle, it will be easier to feed from and interest in the breast may wane.
For help with my mastitis, I would apply a moist warm compress (or take a shower first) and then sit down to nurse my daughter. Although it hurt, I would massage the just behind the lump and gently massage toward my breast to encourage it to move down toward the nipple. it usually only took a whole day of actively massaging during nursing for it to alleviate. Also, i would make sure my daughter's chin was facing the same way as the lump. So if the lump was high up under my arm, I would hold her in the football hold that whole day to nurse and massage with my other hand. i also nursed the sore side first until the lump was gone, expressing any leftover milk from the other breast after she was done nursing.
What my breastfeeding consultant told me was that the most important thing to do is make sure to have the baby nurse first (after applying warm compress) before pumping, so the baby gets and stimulates the breast. Good luck!!