I Have Been On Mirena IUD For About A Year

I have been on Mirena IUD for about a year and a half. I've gained about 5 pounds recently and mi nipples are having some type of discharge that looks milky white in color when I squeeze it. I'm not sure what is wrong? I hope i'm not pregnant! I have had Mirena before about 4 years ago and kept it in for 3 years with no problems. If I am not pregnant, what could be causing discharge from my nipples?