My body is showing signs of pregnancy but the home pregnancy tests show otherwise?

I already have a child and I DO NOT want another one anytime soon. Me and my boyfriend use protection plus I am on implanon. My body has been showing signs of pregnancy: painful breasts, heartburn, mood swings, nausea, frequent urination, enlarged & firm abdomen. I take a home pregnancy test about every other month and they all say negative. Any advice?