I started the pill a week ago

So I started the pill a week ago and I was supposed to get my period but I haven’t yet. My back has hurt really bad and I’ve felt nauseas. I took two pregnancy tests and they both were negative but it’s stressing me out that I still have yet to get my period. Is it normal when starting the pill that you get late? I started spotting a little but then it stopped. I’m just freaked out and worried. Can someone help and give me advice or something?