What Are The Chances Of Pregnancy Despite Having I Pill Post Intercourse?

Hello doctor. This is XYZ last week i.e on 23 january 2015 i had intercourse with my guy n v havnt taken any protection. Aftr that i had taken i pill within 10-12hrs of that. My last periods date was 4 jan acc. To which nxt date shud b before 10th fab as i normaly face late prob. But is it possible that it will get missed or wud get much late this time ???i am damm tensd. And also onething today i m feeling unwell like abit vomiting typ bp low alont with loose motions so is it because of intake of i pill or due to weathr chnge??