To reduce menstrual pain naturally, you can start by taking enough water, stay hydrated always. This helps to filter your body, transport and increase Flood flow through your body. Hence lesser cramps. You can also drink a glass of warm water first thing early in the morning.
You can do stretching and exercising that target the abdominal areas especially just before your period. Yoga is excellent as well. Eat more fruits and healthy drinks. You need to take in substances that can flush through your body.
I've read that when you sit on an empty bucket, it does the trick. I haven't tried it however but I guess there's no harm in trying. Lying down on your stomach for a while helps to relieve pain too. Although these methods do not completely erase the pain, they help a great deal.