A question about the pill, plz help?

Hey Guys

I have been on the Yaz birth control for about 8 months and my doctor changed me to Trifeme. The reason my doctor changed me is because I get really bad stomach pains all through the month, so I have been on about 3 different birth controls to try and stop the pains. Anyway, I started the Trifeme about 9 days ago and the last 7 days I have had my period (they usually only last between 2-4 days) even though I skipped the sugar tablets. The first couple of days it was really light but the last few days it has gotten heavier. Is this normal? I have never had any problems changing birth controls before, i'm just a little worried. I'm 16 if it helps and my periods have always been regular from the very start.

Any help would be appreciated :) Thanks in advance.