Pregnancy On the PIll

HI,
I am on birth control and have been for 5years or so. My brand was switched to a higher dosage about 3 months ago. I take my pills very very consistently. I was wondering if it is possible to fall pregnant and still receive your period during the placebo pills month after month? I have gotten my withdrawl bleeding each month at the correct time but it became a bit lighter after I switched pills. I took a prenancy test after my second month on the new pills and it was NEG!
I know that this all should be proof on no pregnancy but I was wondering if it is possible to still receive your withdrawl bleeding on the pill and still be pregnant and be able to test neg?