lyka23,
With your period ending on 18 Nov, having only one sexual encounter on 20 Nov, and with three negative pregnancy test, the chances of you being pregnant are really pretty low. You are usually the most fertile around 2 weeks after your period, based on the usual 28 day cycle.
If you do not get your period within another week, do another pregnancy test, just be be absolutely sure. Then, if you are not pregnant, you probably have an hormonal imbalance and you should see your gynecologist.
There are many reasons for a woman to have an hormonal imbalance. Some of these causes can produce abdominal and back pain. It usually takes a thorough evaluation by a gynecologist to figure it out.
If the pain is so bad that you can't wait a week, then see your physician as soon as you can.
Good luck.