Yeast is what causes thrush and I am wondering if the reason you are also getting mastitis on top of it is that it is so painful to nurse that maybe you are not nursing or pumping often enough or long enough to empty out and causing blocked ducts. Another thing could be that inadvertently you could be applying pressure to a section of your breast when first putting your baby to the breast because of the initial pain. I have never read any literature or seen any moms over almost 10 years that have had yeast cause mastitis. You might ask you LLL to show you some literature on that and if she has it I would also love to see it.
I in no way have yeast! Believe me with my fourth child I had thrush so many times. He was extremly sensitive to any antibiotics so if I or he had to take them he instantly had thrush. We tried everything natural and nothing worked. Then the doctor kept wanting to use a drug that never worked until finally I and my child took Diflucan and within 3 days it was almost gone. I also had it once with my third child but after a short time got rid of it. Make sure you are completely getting rid of the yeast so that it is not re-infecting you and your baby.
I have had over supply before when working and pumping ( which I did with 4 children full-time) and know I do not have it now. If you do suspect you have over supply and you do not need to pump then stop pumping. Pumping only increases your supply. Make sure you do this gradually for your own comfort. Reduce one pumping every couple of days until you are down to none. Also when you put your baby to the breast as soon as you feel a let-down then remove your baby and have a towel laying in front of you and let the milk gush into the towel. When the let-down stops then put the baby back to the breast. This avoids the gulping and over-abundance of intital milk. With no pumping and using the towel method you should see a difference within a few days.
I know for sure it is some of the funky positions that the twins are doing that is causing the mastitis and I am trying very hard to avoid letting them do these so I do not keep getting it. The worst time is at night I think when I am not really aware of it as much I think but I have been trying to make myself more aware and try not to nurse them at the same time at night. This seems to be helping.
Thanks everyone for all your tips and stories!