Late period from birth control?

Mia

So I started using the birth control pills two months ago. I started my period like three days before the different colored pills (placebo week) actually started. But I lasted until the end of that week. Now it's a month later and I'm three days into that week and I haven't started my period. I've taken a pregnancy test and it says negative. I'm just starting to worry because I'm three days late for my period now. Does anyone know what's going on?