No worries, sweetheart. The birth date of 2/21 and the actual birth date of 2/27 indicate conception at the end of May or in early June, not in early May. Besides everything else, such as, taking Plan B twice, your baby would have been four or five weeks late, which is pretty much not possible. The doctors would never have given you a due date of 2/21 from an ultrasound if you had gotten pregnant on May 3. If you can't believe me, look up some pictures of embryos at 7 weeks GA and 11 weeks GA and you can see there is a major difference in how developed they are. Don't let guilt make you frightened. You're right, it's your boyfriend's baby.
I forgot to add, the may 3rd encounter said he didn’t finish in me. I know that doesn’t matter too much. He didn’t use a condom but he pulled out. And I remember having to clean myself off.