Maybe I am being daft, but why do you think you are pregnant, have you been careless with your pills?
Ok yeah I know you can get pregnant on birth control but usually to be worried as you are there is more to it.
Some birth control pills have the side effect of stopping your periods even on the "inactive days".
How long have you been on birth control for, it sometimes takes a while for your system to settle down.
Some hormone treatments (not sure about birth control) can affect the result of a pregnancy test.
Some birth control pills work by technically making your body think it is pregnant and thereby suppressing ovulation - this could affect the outcome of your test.
So it may be worth seeing a doctor for a quantative blood test to be more accurate.