What Causes Sore Breast?

Hi, have got a very sore (pre period) left breast, with nipple area being VERY sore yesterday. When changing for bed last night, noticed what looked like a 'white head' next to the very centre of the nipple - it popped with no persuasion and a good deal of milky white (non smelling) pus/spot goo came out. Not milky thin, but just like you would get from a disgusting spot - sorry, only way to describe it. Much less sore today, but still some contents if squeezed slightly, which I don't know what to do. Have been diagnosed as pre-menopausal (44 yrs of age) and am due to have merino coil fitted at end of next period due to fluctuating cycle and terribly heavy periods - am therefore due to see gynaecologist in next week/10 days. Obviously am worried about this and it seems just 'another' thing to add to the list - have also got terrible joint/inflammation problems what have been diagnosed as hormone related - am hoping the coil will help with this.