Doctors use your LMP to calculate gestation, it's not when you got pregnant. 24 weeks 6 days is Oct. 15, 2014, close enough.
Assuming your period started on the 15th, you'd have had sex on day 16 of your cycle, OR, using the 17th, you'd have had sex on day 14 of your cycle.
Normally you'll ovulate between days 11-16 of your cycle. Sperm can survive up to about 5 days. The egg is viable for about 48 hours.
"October 31st guy" could be the father.
You'll need a DNA test to confirm paternity.
Ultrasound dating is not exact. A lot depends upon the operators experience. Remember too, fetuses develop at different rates/times, just as babies do. It can easily be off by a week or so.
We can't say for sure when you conceived, nobody can after the fact.
You'll need that DNA test. Sorry.