How Can Paternity Of An Unborn Child Be Confirmed?

Hello. Please don t judge me. I had an early scan in my pregnancy and the doctor told me I was 7 weeks pregnant. I was going to terminate but changed my mind this is how I ended up having a scan. I thought I new who the father was as I have been sleeping with the same person for 2 years trying for a baby. However we broke up in September. I had a silly one night stand with someone on 16th September he said he didn t cum. It didn t last long either! However after this I blanked it out! I got back with my partner and had sex with him around 29th September (roughly, not exact) but we continued to have regular sex. And I haven t been with anyone since. So when I found out I was 7 weeks from the ultrasound. I was happy as I worked out that would of been beginning of October. I had my ultrasound on 22/11/16. I told my partner he is over the moon. But this morning out of no where that one night stand popped into my head Iv tried calculating everything now I m panicking. And don t no what to do. As I m reading it can be 2 weeks different. I need to know ASAP and don t have a midwife appointment for another few weeks! Is it the one night stand from 16th September. Or my partner who I slept with before the 16th and continually from end of September?? Please help and please be honest I m so scared. confused