What Causes Vaginal Bleeding While On 'utovlan' Pills?

I am currently taking utovlan tablets and have been for 13 days now. I took the same tablets last year and they were great!
I am taking these tablets to delay my period and during sex a couple of days ago with my girlfriend i started to bleed. I have not bled since then and it was only that one time. I have skipped a few tablets the past week and i am not sure if this is what has caused the irregular bleeding,