All of the sudden I have breakthrough bleeding? 😕

Mary • 22
I have been on a low-estrogen pill for four months and never had any problems. 
I missed a pill last week, 8 DAYS AGO and I'm still having breakthrough bleeding. Like heavy heavy breakthrough bleeding. I'm only on week two of my pack and I have been taking them regularly since. 
Why is this happening of all of the sudden??? What can I do? Do I need to change to another pill?Â