What Causes Delay In Periods?

i and my girlfriend had intercourse last month(april) it was after her regular periods i used condom but it got cut but still sperms had not come out while intercourse the same happened the next date so i gave her a i pill exactly after 1 weak she got bleeding in the same month(april) for 4 days her regular periods date was on 14th of this month(may) but she has still not got it