this one always seems to come up again and again, doesn't it?
it's just a female ejaculation (i say "just" as in its perfectly normal bodily function, but it's just plain freakin' AWESOME).
run a search on the female ejaculation threads that've been floating around. best thing to do is tell her to let go, it's NOT urine. it'll be clear and odor-less, you'll realize pretty quickly it's nothing bad. although if she's a gusher you may want to lay a towel down unless you want to sleep in the wet spot. my ex would always flood her side and we'd end up snuggling together on my side.
just make sure you let her know and make her comfortable with "releasing", it's pretty much impossible for her to pee. reassure her, tell her you're craving it, whatever it takes to make her more comfortable with it.
edit: i went ahead and did the legwork for ya.
http://www.tfproject.org/tfp/showthread.php?t=60727
http://www.tfproject.org/tfp/showthread.php?t=43152
longest thread:
http://www.tfproject.org/tfp/showthread.php?t=27474