megss

Well, assuming you have a 28 day cycle (average), then today would be cd14 since your AF started on Dec 5th. You might be ovulating right now. Ovualtion happens somewhere in the middle of your cycle and this is when you can get pregnant. Like Vigette said, you can look for signs of ovulation within your body. A few days before and the day of ovulation you will notice that your cervical mucous (discharge), becomes more abundant, wet, stretchy, and clear. This is the stuff that allows sperm to live inside your body for up to 5 days. You may have to check internally to find it. Some women also notice ovulation pains which are low down by your pelvis but are on the right or left side. So the best thing to do would be to start having sex 5 days before you think you'll ovulate and the day of and the day after. (The egg can live for up to 24 hours after it is released.)
Also it is recommended to stay laying flat after sex for about 30 minutes to prevent any of the spermies from leaking out....although most of them will swim up there fairly quickly!
As far as when you can test...you can't get an accurate result until around the time your period is due. Meaning they will be negative even in pregnant women until 10-14 days past ovulation.
Good luck!!!