Advice Please?

I came off Cerelle, the progesterone only pill 4 week ago. I have had unprotected sex since, most days as we are trying for a baby. I haven't had a withdrawal bleed and no AF yet. I also didn't have periods on the pill. Assuming I conceived straight away being 3/4 week ago would a pregnancy test pick this up now? Also had really sensitive nipples the past few days but BFN this morning.