Breakthrough bleeding...is it normal?

I have been on Loestrin 24 for over 6 months now. I have been experiencing some breakthrough bleeding with this last pack. I missed a pill one morning but took it right away that night, then never missed one after that. I have done that before but never had the breakthrough bleeding like I have had this time. At first I thought it was spotting but it was so heavy, dark, and had clots at times that it couldn't have been just spotting. Is that normal? I am supposed to start my period this coming Thursday.