You can use them but you have to remember a major bleed is going to keep bleeding so as one pad becomes full of blood you don't remove it and replace it with another you just put a new one over the last one. With flat 4X4 sterile gauze pads it's not a problem stacking them up but sanitary pads are much thicker and not as wide so stacking several on top of each other is going to create an unstable wound packaging. When this happen it's extremely difficult to apply pressure and it will eventually just come apart which can lead to the wound opening up fully again. Trust me, I've done it more times than I can count and you are way ahead using sterile 4X4's.