Sex with two people in the same month

[Help10123]

So I started my period on the 6th October I then had protected sex on the 9th with a 'one night stand' .
I got back with my ex boyfriend on the 16th October we had unprotected sex numerous of times everyday up until the 30th October. He's addament the baby isn't his but I'm sure it is :( please can someone help me because I cant deal with going through a DNA test to then hurt him if it isn't his I couldn't forgive myself someone help ! The conception calculater said I conceived between the 16 and the 20th October 2016 but I'm still not confident with this information I need a second opinion..
The baby's due 13th July 2017

[Ddc0109]

The conception calculator shows me that for July 13th due date you may have had sex between the 16th - 20th October that caused pregnancy. What was the first day of your last period ?

[AnnieBrooke]

The calculator I use gives an October 20 conception date for a July 13 due date, but that would only be accurate if the ultrasound that told you the estimated due date was no later than about your 6th or 7th week. (The later the ultrasound, the larger the margin for error.)  And there is the five-day length of time sperm can stay in your system to consider, also.  When did you have your first ultrasound, and was it from that ultrasound that you got your due date of July 13?