Who can be the Father of my unborn child?

We don't know the answer to that question.

If there are different men that you are questioning, you will have to wait until the baby is born and have a DNA test done.

my last period was a couple of days before June 25 2011 and My baby due date is April 13 2012 and I had slept with this guy on July 23 2011 could I already have been pregnant at the time or could it be my X baby who I have sex with on the Regular...

I just want your opinion on the information I just posted please...

If your LMP was June 25, that would give you a due date of approximately April 1.

Since your due date is April 13, that means you probably conceived on or around July 21.

Either way, you are going to have to have a DNA test done.