Can you get pregnant on birth control??

I have only been on birth control for the past month. I know that it is not 100% effective, but it is also 99% effective. I am on YASMIN I took my first full month last month. I am on my 2nd week of the 2nd pack. I had about the middle of the ineffective pills when you are suppost to get your period. I only had brown discharge instead of a period, but I started a few days later. I was off my period by about the 5th of JUNE. I had unprotected the 8th of JUNE. This week I have been sick all day like with my first daughter. I have not been able to eat or even look at food. I am so tired and my legs are cramping contantly. I took a pregnancy test Sunday and it was negative. I actually have taken my pills correctly, not missed one either month. How possible is it that I could be pregnant?