What Causes Delayed Periods After Taking I Pill?

Hi doctor I had a unprotected intercourse with my gf 26th oct her period ended on 25 th of oct. We took Ipill and After that her periods was delayed and then she got her perods back on 29th dec. Now she was expecting it on 25th of jan as she had this 25 days cycle but still no chance of periods yet could this be because of I pill.