Missing Period??

Hi, everyone. I recently went off birth control pills at the end of September, after getting my period the fourth week. I took ovulation tests the week of October 10-16, and first got positive results on the 15th and 16th. My husband and I did have intercourse four times that week, twice when ovulating, twice while not. My period is a week late now, but every pregnancy test I've taken has been negative. Has anyone had this happen to them? Any ideas?