If im on birth control but i think im pregnant and i keep taking my pills and buy a pregnancy test will it come out negative because of the pill?

No: If you are pregnant, the pregnancy will produce a hormone, hcg, which is what the pregnancy test looks for.

See below: Conception occurs within day or so of the sex. It takes about 3 weeks for the embryo to make enough HCG for a positive pregnancy test. Blood test may be done a day after a missed period. Home pregnancy tests should be done 4-5 days after the missed period. If the result is negative repeat the test in 4-5 days. Use first morning urine and follow instructions for the test carefully.