2 potential father dilemma driving me sick with worry! Help please

So I have a long term partner of 6 years. we split up in December and I had a one night stand on christmas eve. Me and my partner shortly got back together before new year and it was on February 18th I had my first scan which dated me 6 weeks and one day. i have since had my 12 week that dated me 5 days more giving me a due date of the 8th of October. can someone just clear my head that my long term partner is my babies father? i had bleeding which I thought was a Period on 7th of January