C.O.
If you have the "average" 28 day cycle then you are just at or just past your fertile time if you follow the "rule" that your period starts 14 days after ovulation.
Fo all you know, you could have conceived while typing your question ;) Give it two weeks, if you can manage it, and test then. You will not have had enough time to build up pregnancy hormone if you have concieved at this point.
I also highly reccomend FertilityFriend.com. They have a lot of information and customizable tools for people who are trying to concieve, or who are trying not to concieve
Best of luck to you.