Pill and pregnancy?

I'm a little bit paranoid, I'll admit, even being on the pill and take it religiously. I last had sex without a condom on the 6th Feb and have essentially been quite paranoid about the possibility of being pregnant since.
I've had regular and on time withdrawal bleeds through my placebo weeks since then (three bleeds), and I did a pregnancy test in early March which came back negative, but I have been quite nauseous and fatigued these past few weeks, which has me a little worried.
What are my chances of actually being pregnant even with regular withdrawal bleeds? Should I just do another test to put my mind at ease? 

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