I fill 3-4 large sanitary pads daily with blood during my period for 5 days. i also get dizzy and heart palpitations. is this dangerous?

Yes: You are losing copious amounts of blood with each menstrual cycle resulting in an accumulated blood deficit called anemia. Ultimately, there isn't enough oxygen carrying red blood cells to circulate in your body before you get dizzy on standing or "pass out" See a GYN MD now to treat the anemia and find out why you are bleeding excessively.