What Causes Delay In Periods Inspite Of Taking Ipill?

I had sex on 10th march after dat my girlfriend took ipill within one hou even though we were using protection... after 7days she had some red discharge for 3days and now she haven t accuried her periods dismonth.her date was 4th of dis month... she is 10days late.. is dis the side effect of the pill..we had HPT but it showed negative .plz answer