What can I do when I'm lightheaded during menses, from very heavy flow.?

Light headed. You may have severe anemia from blood loss. This can lead to low blood pressure especially when standing up or in the shower. Have your doctor order a CBC to check for anemia.
Also... Agree with need to check CBC for anemia, and also check TSH for thyroid dysfunction. You can also start taking on otc iron supplement 1-2 times per day. You will also need evaluation of uterus with sonogram, and possibly endometrial biopsy if the lining is too thickened, but you definitely also need treatment to control the bleeding. Many med options, if u plan 2 have kids; if not, more options.
Blood work. Visit your doctor to check for signs of anemia, or low blood counts. Your doctor will check your pulse, measure your blood pressure, and probably suggest some blood work. It sounds like you might have anemia. The other very important thing is a thorough pelvic exam to make sure that you are not having abnormal bleeding.