Late Period

Hi. My period is 4 days late, and I've been really stressed about it lately. I am sexually active, but I've taken 3 pregnancy tests that have all come out negative. Is there a way to induce my period? Also, I was recently prescribed birth control pills by my doctor. She said that I can start taking them this Sunday if I get my period before then. What do I do if I don't get my period by then?