What Causes Vaginal Bleeding After Starting Birth Control Pills?

Hey I ve just started with birth control and the nurse told me to now start with them until I got my period.... So I waited but I I didn t get my period so I didn t start with them. I had sex without any kind of protection and took a day after pill... Then the nurse told me to imedieatly start with my birth control pills but still use protection for a week! So now I ve used my pills for like 3 Days and I started to bleed! A lot.. I think I just got my period but how is that even possible? I m so scared