My doctor put me on Lo Ovral (generic form) to control bleeding and endo, and I did the exact same thing as you the week before my period, only after sex. The third pack of pills, it didn't happen and hasn't since. Some pill have a greater % of bleeding rates, you probably need a stronger pill. I always hated TRI type pills anyway! Monophasic has been the best for me.

I am switching pills, not because of bleeding, because Lo Ovral has worked great at controling my periods and pain, but because it is making me a
b!@*#! I am going to try Mircette (generic) it has a low % of breakthru bleeding. If that doesn't work for me, I am going to do the Mirena.