Help! I had sex 2 weeks apart now I’m pregnant.

i Hope no one will Judge me. I’m 25 and I was in a relationship for 3 years, it ended in jan this year. Me and my partner had sex around the 2nd Feb and definitely on the 9th. I can’t remember my last period and they are never regular. I also had sex with someone on the 13th Feb (stupid I know) i Went for an ultrasound on the 21st of March and they told me I was 6 weeks and 5 days which takes me back to the 2nd of Feb. who do you think the father is? My mum said I couldn’t get pregnant with  second guy before I had sex with him but I’m not convinced. I do realise il have to do a DNA test but please can someone give me advice? I am so so scared