What are the best ways to aleviate a stomach ache caused by too much chocolate?

Many things. Many people have trouble with gastritis or reflux, due to hyperacidity or making too much acid. It can be due to a medical problem like zollinger-ellison syndrome, but most are due to the food you eat, or when you eat it. Numerous medications will help with it. Talk to your doctor about it.
Chocolate hurts! Chocolate contributes to stomach aches because: 1) theobromine in chocolate is a caffeine-derivative; 2) the calcium in the milk of mik chocolate stimulates gastrin to turn up acid production; & 3) the fat in chocolate delays stomach emptying. Moderation is key here, so enjoy chocolate but not to the point of promoting "stomach aches" & avoid eating chocolate with foods that upset your stomach.
Moderation is key. Moderation is key when it comes to dietary intolerances. Chocolate promotes dyspepsia 3 ways: 1) high fat content of chocolate delays stomach emptying so foods & acid stay in contact with stomach longer & reflux into the esophagus (gerd); 2) calcium in "milk" chocolate stimulates gastrin, a hormone that turns up acid production; 3) theobromine relaxes the lower esophageal sphincter causing gerd.