When i drink alcohol, one drink, my stomach starts hurting. what could that be ?

Gastritis: This sounds like gastritis, a general term for stomach irritation. Gastritis may occur without a more definitive diagnosis, or may represent underlying issues e.g. Ulcers, which can be dangerous. The general rule "if it hurts, don't do it" applies here- people with gastritis should avoid alcohol and other identified triggers, and seek help if symptoms persist or worsen.

Stomach pain: It's difficult to say. We need a lot more information. Do you have other medical conditions ? Do you take medication ? Could you drink alcohol before this? Do you have stomach pain under any other conditions? In short you need evaluation.