i think i have the same thing... i have just noticed it today, im 21 and have a 6 week old baby who is staying at my mothers house at the moment because i think i have a cold or the flu or something so thismorning i started expressing and when i got to my left boob after 15mins of expressing this thick yellowish puss like stuff started to come out.. its not milk its totally different.. im wondering if its because im sick... i have no idea.. its a bit scary though.. but it seems quite a few people get this problem too. have you had the dr check yours yet?? if so what did they say?? i have an appointment with my dr in 2weeks time for my sons check up appointment so i'll mention something then..
If this is truly pus rather than just a breast discharge (pus would have an odor) then you should see your Dr. ... if there is an infection then you would need to be treated with some type of antibiotic. This would no doubt be some problem involviing the milk ducts .... see you Dr. for an examination and any test or treatment that he/she thinks is appropriate for your problem. Regards ....