Antibiotics and Yaz...crazy symptoms...any advice?

I have a 2 1/2 year old daughter and a 1 year old son. I have been on Yaz since March, and up until recently I haven't had any issues. I had an upper respiratory infection a few weeks ago, and the dr prescribed me a strong antibiotic. She asked where I was in my pill pack (week 3), and she said that the antibiotic wouldn't affect anything, so I didn't need to use back up protection with my husband. A few days before my period was supposed to start, I bled lightly for 2 days. When my period was supposed to start, I didn't bleed at all. I wasn't worried until I started my next pill pack. I've been having all of the pregnancy symptoms I had with my second child. I took a pregnancy test, which was negative. Has anyone else gone through this? Should I go back to the dr and get a blood test done, or could I just be being paranoid? I just hate to continue taking my birth control if there is a possibility that I'm pregnant. Thanks so much for the help!