Period over 10 days?

Hey, I'm 16 and I've been bleeding since I was 13. It's been fairly regular for a year now (my cycle is always a few days longer than a month).

I started taking the pill about a week ago. For about three days now, I've been feeling nauseous, especially in the evenings, and I have hardly any appetite. My energy level is lower than usual, but that could also be due to the current climate.

I took the pill on the second day of my period, or rather I got it the day before at 12 o'clock at night and then forgot to take it straight away, so I took the first one the next day.

My bleeding usually lasts 4-7 days, but now, after 10 days, it still hasn't stopped. The 8th and 9th days were just dark spotting from older blood, but today there was more fresh blood.

I've read that this can be normal, especially after starting the pill, but if the bleeding stays the same or gets heavier, then it's already continuous bleeding.

Does anyone know this? Why is this happening? What should I do?