"I had sex May 28th had my period May 31st then had sex on June 21st and 25th .. took a pregnancy test July 2nd came out positive , didn't have my first ultrasound untill almost 19 weeks which said I was due Feb 26.Due to my period im due March 7th ..which one is correct ?"

Gestational dating is all Best Guess based on available information.
If there is a paternity concern then a DNA test will need to be conducted after the birth.

"I had sex May 28th had my period May 31st then had sex on June 21st and 25th .. took a pregnancy test July 2nd came out positive , didn't have my first ultrasound untill almost 19 weeks which said I was due Feb 26.Due to my period im due March 7th ..which one is correct ?"

There is no way to know - if there are two different men involved DNA when the child is born will prove paternity.

The later the ultrasound the less accurate it is, but if you had your period after you had sex on the 28th, its unlikely the person on the 28th is the father. So I would say its more likely the June dates.