I have a Medela Lactina and am currently using 21mm shields. I had been using 27mm shields (same size I used w DS 4yrs ago and had no problems) but was experiencing nipple redness and soreness and discomfort upon starting pumping. I work 3 nights a week and noticed that my nipples were sore after working a few nights, then would feel fine after a few nights off. I did a little research and found my shields were WAY too big. So I tried 24mm, still too big. Now I am using 21mm and they seem to fit my nipples the best. I am also trying to keep the suction lower while I pump to see if that helps but I'm not getting as much milk with it lower. I notice the tips of my nipples look slightly bruised sometimes when I'm done and usually a little swollen. Any ideas?