Why is it that my IUD gives me such sharp pain sometimes?

Might not be IUD. If the iud is in the proper place, it normally won't hurt. You can check to see if your strings are in the normal place. Your doctor can do an exam and order and ultrasound to check placement also. Lower abdominal pain can also be caused by problems with the bowels, uterus, ovaries, and uterine lining (called endometriosis), i would recommend seeing your doctor for evaluation of your pain.
Check placement. With your doctor, any recurring pain particularly during sex, jogging, etc. Should be worked up by your doctor.