The truth is you'll need to get a paternity dna test. If you sleep with multiple partners in a short amount of time, the odds are it could be either one.
Statistically, your risk of getting pregnant just after your period is quite low. However, that is not to say your ex isn't the baby's father.
No one can ever tell you the exact date of conception, even doctors or ultrasound techs - unless you only had sex one single time that month you couldn't know the 'exact' date.
Generally, if you know the first date of your last period and the normal length of your periods, you would know approximate dates of ovulation. That is how the doctors do it.
Good luck.