My Girlfriends Last Period Was Near The End Of January.

My girlfriends last period was near the end of January. In February we ended up having unprotected sex on the ninth. I pulled out before ejaculating but purchased and had her take the Plan B pill two hours later just to be safe. A week after taking the pill she began to bleed lighty/spot. Its now late March and she still hasn't had her period since she took Plan B pill but a couple days ago she felt like it was about to begin her period but then she ended up just "spotting" again for two days. After that I had her take three pregnancy tests and they were all negative. Is the Plan B pill causing her period to be late?