ash Scans can be wrong but I think become slightly more certain as weeks go by, so it might be worth checking with them again. In the meantime, have you crunched your dates as much as you can?
Sperm can live for up to five days, but I think most people work on about 3 days. So to be pg it must come from unprotected sex that happened about 5 days before ovulation or possibly in the days leading up to ovulation. It is possible to get pregnant if you have sex one day after ovulation, but this is less common.
Also, implantation of a fertilised egg takes 7-10 days and I think in most cases people only start to feel pg symptoms after implantation when hcg levels start to rise. I'm not sure your body really knows you are pg until implantation happens. So .... if you had sex with the guy on holiday and then felt odd about three days later, it suggests that it might be from sex much earlier.
Finally, how long after your period did you first get a positive test? You can get a positive test as early as 10 days after ovulation (I assume if the egg implanted at around day 7), but this is not as common as people think. Its more common to get a positive result when your period is first due or in the days immediately before your period is due.
If I were you I'd try and draw all this on a calendar and then work out possibilities if you factor in times with both men.
Hopefully someone will come along and set me straight if I've got it wrong.
But ultimately the scans are probably your best guide if you need to make critical decisions, and a dna test afterwards to be totally sure.
Good luck.