conflicting pregnancy tests

i have also had nausea, sore breast, and frequent urination.

Birth control would not be messing the test up. Pregnancy tests (both urine and blood) test for the presence of hcg, a hormone produced during pregnancy. I've heard of people having positive urine tests and negative blood tests and turning up not pregnant. The other thing is, did you get a quantitative or a qualitative blood test? A quantitative tells you the exact amount of hcg in your blood. The average non-pregnant woman will have less than 5mIU/mL. If you have over that amount, there is a good chance you may be pregnant. A qualitative blood test gives you a yes/no answer, and typically will only say you are pregnant if you test 25 mIU/mL or higher.

Starting birth control after just having a child could be causing your nausea and sore breasts.

If you want to know for absolute sure, I would recommend you get a blood pregnancy test again. Ask for a quantitative hcg test - this will tell you the exact amount of hcg in your blood and is the most accurate test for determining pregnancy.