Can you please tell me?

My.breast are hurtin really bad and are a lil fuller then normal. I can't be pregnant I have the IUD marina. I am only 19 so could they jus be growin or really filling.out from breatfeedin my daughter last year she is now 2yrs old and I breast fed her for 6months. I took a pregnancy test and it said neg. Can someone please tell what's goin on