Haiii... I have similar symptoms and our dates kind of match as well... Would I be right in reading your last period as 20th May 2011 rather than 20th March 2011...
My last period was on the 23rd May 2011 and I am on a 30 day cycle.. Called my gyne because of the symptoms and was advised to wait until my next period date to get an accurate result. Is now waiting for the 23rd June 2011 to get a Beeta Blood Test... Fingers crossed for both of us!!!
Symptoms are completely unreliable and the only way to know if you are pregnant or not is by a pregnancy test, taken either at home or at your doctor's office. If the test was taken correctly in the correct time frame(at least 2 weeks after sex and following directions on the box), then I would trust the test. Though if you still feel you may be pregnant, stop taking the pill and wait until your period is supposed to start. If you want to have sex, use condoms. Then if your period doesn't come, take another home pregnancy test. Good luck.