The best product that I have personally used recently, is ThermaCare Menstrual Heat Wraps. They're relatively inexpensive, last about 8 hrs., and can be worn underneath clothing ~ great for busy days on the go. Otherwise, you can use moist heat, such as a damp washcloth rinsed in hot water, squeeze out excess water, then apply to your abdomen. Another over-the-counter pain reliever is Midol; you can also use maximum strength Ibuprofen instead.
Good Luck!