How To Determine The Paternity Of The Fetus?

I am 15 wks pregnant to my surprise, I split with my long term partner on the 18th November started a new relationship on the 2nd of december , My Lmp and dating scan have matched exactly , my periods are regular and I have a normal cycle , my due date is the 31st august is there any chance it could be my x partners baby or am I just over reacting