I was last in mexico four days ago. i avoided tap water, but ate twice from vendors on the street. i have diarrhea and frequent stomach pain. normal?

Not normal: Food from vendors can be contaminated with bacteria that can cause diarrhea and abdminal discomfort. A health care provider can help, often without a lot of tests.

Travelers Diarrhea: Sounds like you probably don't need anything except to keep well hydrated to keep up with the diarrhea. Most cases of td resolve in 1-2 days. Some may need antibiotic for travelers diarrhea.(probably not if young and healthy). It is also helpful to take some probiotics to help recolonize your gut with normal flora.