My friend and I had sex on the 29th of last month. Her last period started on the 15th of last month. We used a condom the whole time, but I was worried there could have been a leak, and I got her an I-pill within 18 hours of intercourse. She has had no bleeding whatsoever since then and has not even had her period yet. Is there anything to worry about? She normally gets her period in 28 days. Please help.