I have had something similar happening for years. Both nipples were inverted from birth however. About nine months prior to my first pregnancy a lump formed under my nipple which was extremely painful, and the area around the areola turned red(cellulitis) and hot to touch. It went away after lots of different types of antibiotics and many trips to the ER, specialists and ultrasounds. It however came back while I was pregnant and again while I was nursing, however went away on it's own each time. One time it surfaced and exploded. In the ultrasound they saw a pus pocket but it was hard and they were unable to drain it. I went through the whole gamut of testing; paegets disease, breast cancer, mastitus. The last time it happened I was in Denver visiting family and was sent to a specialist who had luckily just gone to a seminar about this very problem. It is very rare however more common with inverted nipples. It is called a sub-areola abcess. More like a GIANT cystic zit under the nipple. I had yeast problems, cracked nipples etc. It can be treated with antibiotics, however It usually comes back. It is when bacteria gets into a duct and travels all the way down to the root and gets stuck there. Baisically the duct opening at the skin level is also inverted and when the bacteria got in it was trapped and could only go down, when usually a woman's body automatically pushes it out. The only real way to get rid of it is through surgery where they pull out the duct. It has a two week recovery time, and is really painful, however not as painful as the abcess so in the end it was worth it. After the surgery The only thing I was upset about was that my nipple popped out, and is no longer inverted. Strange, but I liked my inverted nipples. I am currently dealing with it again on the other side, which I was told would most likely happen, and Im going to see a surgeon next week. Im planning on surgery in August when I go back to Denver because I will have more help with the kids for afterwards (I can't pick either of them up for two weeks, or till the drain is removed). I was also terrified when going in for the ultrasound, however there are many things it could be without having to be breast cancer. Keep in mind though that despite all of the advice given here, no one can make a diagnosis on a website. This was just my experience with something that sounds similar. Im keeping my fingers crossed for your sister.