Dude, you have experienced a truly wonderful result of lovemaking. You and she are so HOT together, and you are giving it so good to her, that she is squirting! Her vagina has been so perfectly stimulated, in all the right places, that her orgasmic musculature is reverberating to your stroking with pulses of joy! (Men get an erection in ONE place, whereas women are fortunate enough to get an erection in at least 5 places, if her partner has done his job.) Sometimes the pulse is filled with her juices, and you get wet all over the front of your groin, from belly to thighs. Sometimes the pulse is filled with air, and you hear what sounds like a fart. But, it's all due to lovemaking with a fabulous attraction and desire for one another, and the physical ability to carry it out in equally-matched sex. That being said, the best sex is first triggered in the brain, of course.