I've also been with a virgin, and I'd bet that it's the urination issue because she went through it, too.
In the beginning, in the middle of having sex, she would ask to go to the bathroom. I would say, "No," jokingly, and then let her go. Another time she felt the urge, I would tell her that if she really did pee, it would be okay. "It'd be kind of a nice, warm feeling down there, I guess...," I would joke.
After a week or so, she stopped having the urge to halt the session and visit the bathroom. She now stops the session if she actually does need to go to the bathroom.
She realized that the peeing sensation was nothing but an incoming orgasm. I wouldn't even worry about it, bro. Just let her go to the bathroom beforehand if she thinks it will help.
Since she's a virgin, the more important thing is to let her know that everything is going normally, and that her body is perfect through your eyes, etc. Be completely positive and make sure she's comfortable.