menstrual cycle & possibly pregnant?

I had unprotected sex with my boyfriend on december 31st. My period was late, but that usually happens to me. We then had protected* sex on january 18th. However there was a small problem & I took a plan B pill at 8:40pm & then one on january 19th at 8:40 am. I still haven't gotten my period from BEFORE the pill. I am now 17 days late. I keep waiting because I've read that the pill can cause your period to be more than a week late. IS THIS COMMON? I plan to take a test, but I'm stressed about the whole situation, & maybe this is what is causing the delay.