Hi.
Listen, my boyfriend and I love to do it from behind.
And yes, I know that feeling of wanting to pee.
http://www.healthboards.com/ubb/redface.gif)
Pressure on that area can cause that feeling. It doesn't mean you're really gonna pee, necesarily, but it is distracting.
Just go to the bathroom just before to make sure your bladder is empty.
And the feeling is intense because of the angle.
I highly recommend that position, and I think you should try it till the end. And you can use your hand too, to help yourself along.
This may be a different orgasm than you're used to, because maybe your other orgasms were clitoral and this is more of a vaginal one.
And if you're still to worried about peein, how about trying it in the shower? If you can get in that position in the shower, that is... That way, even if you have a little "accident", no one will notice. But again, I really don't think you will actually pee, especially if you went before.
http://www.healthboards.com/ubb/redface.gif) Good luck!