Lovely42. The fertile window is three days before ovulation and one day after. Semen can lives in the female body for five days while the ovum (female egg) lives for up to 24 hours.
What this bit of information tells you is that if your BF ejaculated into you around the time you should ovulate in that three day window,, the chance of pregnancy happening is pretty good.
It takes an 7-12 days to implant inside the uterus..some people claim that they get a HPT in the 6DPO window but this is not possible, chance are they ovulated a bit earlier than they thought they did. So it is possible to get a HPT negative because the body is not producing enough HCG in the blood stream to be detectable. This only happen once implantation has taken place and then it starts to double every 24 hours. Also make sure to find what level or how sensitive the HPT is as this could assist a great deal. I would suggest that you wait a few more days to ensure the egg has indeed implanted and retest using FMU (First Morning Urine).
I wish you all the best and keep us posted.