What do you do for stomach pains resulting from orally taking too much arnica?

Stop taking Arnica!! I assume you're referring to arnica herb (http://umm.Edu/health/medical/altmed/herb/arnica). So when it comes to dietary supplements, if they bother you in some way, stop taking them. If you know that you've been taking too much arnica (or whatever else), just cut your dose. Of course, if you believe side effect is due to prescription medication, discuss side effects w/family doc & prepare option.