Now this might be tmi, but I'm not sure if this a normal thing for vaginal muscles. I use a menstrual cup, and when it doesn't open up completely, I can expand those muscles so that my vagina fills up like a balloon. If I do this and put a finger in my vagina it feels like a mini balloon- the kind to make balloon animals out of. I have stubborn belly fat, and when I do this the fat kind of bunches up all in one place in my abdomen. I can then expel all the air out when I want. Not everyone can do this right? I feel kinda weird XP. I think I remember reading somewhere that there was a woman who could stop her period because she had such great control of her muscles, but I'm not sure if I remember this correctly.