What Causes Breast Nipple Discharge?

It is now September 12th 2011 I am 19 years old. I had a miscarriage last year of may 5th 2010. Its now September 12th and im trying to concieve and get pregnant i've been trying for the past 4 months now since June 19th 2011.& just the other day my fiance was just playing around with my nipples and starting squeezing them to see if anything would come out and white/clear looking milk came out of my left nipple but not my right nipple!! I took a pregnant test on September 12th 2011 just today and it came back negative. Im really worried and curious what this could be? Could someone help me out with the best advice on what you think it could be. Email me at YYYY@YYYY thanks have a nice day. Love Amanda -