What is the length of your cycle? Based on a 28 day cycle - If the first day of your LMP was 20 Dec, you should have ovulated around 2nd Jan, and your period due around 16 Jan. So the light bleeding on 12 Jan could have been implantation bleeding. However you should have been able to get a positive HPT by the 30th when you tested.
It is also possible that you ovulated later (stress, illness, etc can affect ovulation time) so perhaps the light bleeding was from ovulation.
If you are unsure and feel as though you may pg you may want to ask your doctor to do a blood test.
Good Luck, I hope you get the answers you are looking for. Update us when you can
Not everyone's cycle is text book like that and cycles can vary month to month.