What Causes Heavy Vaginal Discharge While On Birth Control Pills?

My 15 yr. old daughter has been on tri-sprintec since February 20, 2015. She had one normal period in March. She has been bleeding since April 5. She did have the flu for 3 days (4/5 thru 4/8). The bleeding has been heavier since Monday, 4/13. I called her doctor today, they changed her to Enskyce. She took the pill about 5:00 this evening. She has bled through 2 super tampons since 4:00 today. Should we be worried? Is this ok for someone starting birth control. She will be 16 on June 8