Well, you just found out yesterday... so in a way, you have to mourn the loss of your "feeling" it was a boy. In a way, it is like parents who find out that their child has Down's Syndrome. They will love their child, but all the hopes and dreams and plans for their child are suddenly changed. In that respect, you are going through the same thing. No football in your child's future, no blue boy's nursery, no future Daughter in law, etc. So it is okay to "mourn" the loss of these things you anticipated. It will pass and you will move on.
That said, we were told that our first baby was a girl. I "knew" it was a girl all along. Well... they were wrong. And so was I. Granted, the ultrasound was much earlier in my pregnancy, and technology 12 years ago was not what it is these days... but there are errors and until the baby is delivered you won't know for absolute complete certainty.
Your feelings right now are normal. Whether you have a girl or a boy, you will love your child. And none of it seems "real" until they are delivered! lol Once you are holding your baby in your arms, you will be fine..