Not knowing who baby's father is

Sorry to ask but I've been so worried these past few weeks. i slept with my ex parter up until the 7th of Jan But then I also slept with someone on the 10th of jan.. My last period ended on the 21st of December only lasting 5 days but everyone says you ovulate 14 days after last period had started and that brings me to the 27th december- 1st of jan! I'm just wondering what people think? Do people think it's my ex's baby or my new partners.. Thanks in advance