What Causes A Delayed Period While On Birth Control?

Should my period have started by now?
First, I'm on birth control and have been for over a year now. I went to the doctor last Friday because I felt sick, turns out I had a sinus infection so my doctor proscribed me an antibiotic (Amoxicillin and clavulanate potassium tablets) which I'm suppose to take one tablet two times a day. I didn't start the medicine directly on Friday, instead I started them the next morning on Saturday (I took one in the morning and that night). Well, Sunday (yesterday) me and my boyfriend had sex without using a condom. I didn't realize till after that my antibiotics can affect my birth control and my period was suppose to start yesterday! Because it was my first pill of my last week! So now I'm one day late and really worried. What do you think I should do? I was thinking of buying Plan B pill.