I have anywhere from a 28-31 day cycle. My lmp was around may 23 buy my dr has it at may 21. I had 2 early ultrasound at diff hospitals that gave me the exact same estimate of how far along I was at 7 and 11 weeks. It happened (the one night stand) around the 16 however he didn't ejaculate and I was having sex with my bf most days of the month...
A few questions: Do you have regular cycles? How long are they, and when in your cycle did you have the one-night stand?
It can take a normal, healthy couple up to a year to conceive, so yes, 9 months is normal...however, if you also had a one-night stand, it would be hard rule him out as the potential father. Sperm can live for a few days in your reproductive tract if it happened before ovulation, and also there is always a chance that you ovulated a few days earlier/later that expected...unless you've been tracking your basal body temperature and/or used ovulation tests (if so, it would be easier to rule out). I hope everything works out for you.