Chamomile can help ease the pain of a stomach ache by working as an anti-inflammatory (for example the lining of the stomach can become inflamed as a result common gastritis, caused by bacteria) and by relaxing the smooth muscle of the upper digestive track.
Rice water is exactly what it sounds like-the water left-over after you cook rice. It acts a demulcent, meaning a substance that relieves inflammation by forming a sort of soothing barrier over a membrane, in this case, the lining of your stomach.
Fresh peppermint tea (or just peppermint tea in general) can help relax stomach muscles. It also helps improve the flow of bile, which helps you digest properly. This is especially useful if suffering from indigestion or gas/bloating.
Lemon water, if your issue is indigestion, helps a stomachache. The high acidity level stimulates the production of hydrochloric acid, which breaks down our food. By upping the amount of HCL being produced, you help move digestion along at healthy pace.