The 1st day of your last period was on May 30. That usually means that you ovulated and were fertile and capable of becoming pregnant 14 days after May 30, on June 13. You would be fertile for about only 5 days, beginning on June 13. That would be only time you could get pregnant in that cycle.
You had unprotected sex with Guy A on June 5, but he couldn't be the the father of your child - you weren't fertile then and you had .finished your period only 2 days before.
You had unprotected sex with Guy B on June 16, during the time you were fertile. He is most likely the father of your child.
It beats me how you could have gotten a positive pregnancy test on June 28 - that's only 12 days later, at the most; and your HCG levels wouldn't have been high enough for a pregnancy test to detect - not unless you were ALREADY pregnant before you had sex with either of those two guys.To be absolutely certain of the father, you're going to need a DNA test.