I found out on Feb. 15 that I was 5 to 6 weeks pregnant. I don't know my last period. But I had sex with my ex on Jan 23. Can I be pregnant from him?

Anything is possible. You have to remember that when figuring out gestation its mostly a guess. Everyone is different. So its possible. You don't have to answer this. But do you have a list in mind? If not you better make one while names are still fresh in your head for later.

Not as likely, but within possibility. There is no way to come up with an exact date, all numbers as to how mnay weeks are estimates from dates of last periold that you gave doctor and/or from size of fetus

So yes, if during that several week periold you had sex with more than one person, you will have to get a DNA test after baby is born to be sure

Wait until your first ultrasound. They can normally generalize how far along you are, but even that is off by 2 weeks give or take.
My husband and I are in a similar boat. I haven't had a period in 2 years, and we're basing our due date simply off the ultrasound.

Just be prepared as soon as you hit 37 weeks. You never know

And even the "guess" from the ultrasound is merely based on average size of the baby at certain weeks. It can be off enough that you will not know for sure, until you get a DNA test after child is born