What Causes A Delayed Period After Taking Next Choice Pill?

Hey, I had my period on June 1st, on June 7th me and BF had unprotected sex. I I freaked and took next choice on June 9th. After much talking we decided to try to conceive. I bleed about 7 days after taking the pill. My period is now 13 days late. But I keep getting negative results from HPT. Could I be pregnant or is it just from the pill? Thank you!