I have been having severe muscle cramping several times a day and at night. What can be causing this?

Number of things. Dr. Kopes-kerr is right, statins are a common cause of leg pain. Certainly, phlebitis or even a DVT can cause leg muscle cramping, as dr. Namey said too. One of the most common symptoms of varicose veins, though, is leg muscle cramping during the night and day. If you have veins you can see in your legs, see a vein specialist (phlebologist) to get them checked out.
Muscle cramps. Assuming that you are not suffering from heat exhaustion/heat stroke, i would consider the possibility of an adverse medication effect. The "statin" medications for cholesterol are particularly known for causing exactly this effect. Other medications like major tranquilizers or antihistamines can also sometimes do this.
Depends! Depends on which muscles. If legs, could be low potassium, magnesium, calcium, and iron deficiency. There are other cuases: phlebitis, but this would be rare in anywhere but the lower legs to mid-thigh, and rare in a very young person! if problems persist, see your dr!