Going from barely wet to a waterfall??! Why

So whenever my bf and I do it in normal positions (missionary, cowgirl, doggy, etc) I'm turned on and wet but just enough for there to be friction. BUT whenever I lay on my back and he lifts my hips off the bed and fucks me like that I get SO WET SO FAST to the point where it's flowing out of me. This is the only position where I feel the most pleasure and it feels sooo good (I guess cause that's the "g spot") but I don't understand why so much comes out of me. Its not like I've orgasmed yet bc I never have before. It's kind of embarrassing bc the beds always soaked after. Any ideas? Anything? Questions, comments?