What can be the source of a bitter aftertaste that I am experiencing
after a meal recently? The taste lasts for a few minutes only, but
returns after I eat again. Perhaps it has to do with a diet I have
just started in which I don't take any sugar or white flour and avoid
eating fats and carbohydrates together in a single meal.

No one can diagnose you over the Internet, of course.
Gastro-Esophageal Reflux Disease is one of many possibilities.

"Other symptoms of reflux include the following:

Regurgitation of food or fluid into the mouth. This often leaves a
bitter taste in the mouth. You may not be aware of bringing up
anything, just a bitter taste which appears in your mouth."

http://www.mydr.com.au/default.asp?article=3352

Hi Yoyogam, I am not a doctor but I had similar symptoms in the past.
There was a sensation like hot pepper in the mouth. It was caused by
too much acid in the stomach and was relieved by my doctor prescribing
a neutron pump inhibitor called Maxor. I have also tried a natural
diet with no processed foods and plenty of water with great success.
Please see your doctor for advice. Hope this helps.

A friend of mine has had relief using green tea extract.