What is that fish smelling secretion that is coming from my female dogs vagina?

My female Blue Nose Pitbull dog has been spayed for a year and a half. Sometimes when she lies down she will secrete a small amount of this putrid smelling liquid. I often find it on her bed. What the heck is that stuff and how can i make it stop? it is nasty and makes me gag. Worst of all my non neutered Male Blue Nose Pittbull always goes over to it sniffs, and then starts licking it. Then his forehead does this weird throbbing thing and his mouth starts clapping together.The scent is also in her pee because the male reacts to when she pees the same way to when its on her bed. Sometimes the male will even lick her vag directly and then his head will start throbbing. My question is what the hell is that funkified liquid from my female and why does my male dog throb his head when he licks it?