I had sex on16day of dec and as early as end of the same month 30, 31, 1, 2, 3 my period came is there possibilities that I can b pregnant?

Yes. There is always a possibility that you could be pregnant. It sounds like you had intercourse and then started your menstrual cycle days later. Some people have spotting or bleeding during pregnancy, so if you think you might be pregnant you should take a test.