All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
If the second bleeding that you experienced on the 20th of December occurred during the time of your regular period, it is considered menstrual bleeding, otherwise, it is considered Istihaadhah (vaginal bleeding outside the menstrual period) in continuation of the previous bleeding on the 15th of December. If it was menstrual bleeding, you would have experienced three menstrual bleedings before your marriage and accordingly, your marriage contract would be valid. If it was Istihaadah, your marriage is invalid and it is incumbent on you to refrain from engaging in intimacy with your husband until you renew the marriage contract.
Allah knows best.