Who is baby daddy sorry kinda long

So my last period was may 28th had sex the 8th well 2am on the 9th he was a rebound let's name him Alex hahah and I got with and old boyfriend let's name him Thomas on the 14th well that afternoon due to my last period I ovulated 10 or 11 but when I went to my first ultrasound baby was measuring four days smaller but dr going off of last period I think it's Alex (not his actual name ) but now worried it could be Thomas (not his actual name)

Oh DNA test will happen for one of them other possible one Thomas blocked me