Your period was expected on Dec
. 29th. On Dec. 31st you had for one time. You had sex on 3rd
You had some bleeding 2 hrs after sex when you went to the rest room which did not recur.
Menstrual Bleeding need not continue from day one. So probable cause,
1) Your cycle began on Dec 31st & did not continue
2) Your cycle started after sex on 3rd Jan.
3) Bleeding due to force, unnoticed immediately after sex, but noticed by you when you went to the rest room (usually clot after 2 hrs - but mixed with water it may look like fresh blood)
Wait & observe for few more days