Don't want to waste my FRER!

First day of last cycle was the 21st with light spotting beforehand, making the last fertile window between Dec. 1st-4th. BD'd on the 5th, and was due for AF on the 14th. Took a test, which was negative, and have had some cramping and nausea (but I don't know if it's psychological ;). No AF to date (5 days late), and as a result of such my fertile days have shifted, making the 5th (when I BD'd) my highest fertile day. When should I take my other test? :)