Hard ball like thing under my skin?

So I had surgery recently on the 3rd of February. I developed this ball 2 days or so after my stitches got taken out. It's hard like a rock and is small oval shaped. It hurts to move it sometimes. Yesterday it felt kind of mushy, then it slowly formed to a rock hard oval again. It's quite sensitive to the touch & i don't know what to do. My doctor said put warmth on just that area, which has not helped AT ALL. Help?