Bit of background information: My last period was on January 18th of this year. I'm semi-regular, sometimes it comes later than expected, sometimes earlier.

Basically, I had unprotected sex last Sunday, February 12th, and took Next Choice three days later on the 15th. I took a pregnancy test this past Saturday, February 18th, and it was negative.

My question is, (1) if I have a negative pregnancy test but my period is seemingly late, what is the cause? My boobs are a little tender, as well, but that can be an indicator of both pregnancy and the start of a menstrual cycle. Also, (2) when should I take another pregnancy test again? And (3) Is this normal, or is there a possibility that the pregnancy test failed and I could be pregnant?