Breakthough bleeding concern,help!?

Ok so my gf has been on birth control for almost 6 months now. She takes it at the same time everyday within 10 minutes and has never missed a pill. Today marks her 19th day of her cycle. We were doing it today and when I pulled out to switch positions I noticed I had dark red blood on my .( we always use a condom but I will put it in a few times without one on before we start sometimes. This is when it happened.) we stopped doing it and looked at her underwear she had taken off and there was a little blood on her underwear too. Do u think this is breakthrough bleeding or implantation bleeding?