The stomach aches and sleep issues are real physical responses to anxiety, so just reassuring her won't cut it. What actually helps is breaking down the test into smaller, less scary pieces. Have her study in short sessions over several days instead of cramming the night before - like 20 minutes a day. That alone reduces a lot of anxiety because her brain isn't overloaded and she actually remembers stuff better. The night before a test, skip studying entirely and do something calm together instead. Make it boring and normal, not a big deal.
Also talk to her about what specifically worries her. Is she afraid of failing, or getting a bad grade, or looking stupid? Once you know, you can address the actual fear. Sometimes kids catastrophize - they think one bad test means they're dumb forever, which is obviously not true but feels real to them. Help her reality-check: "You got a B on the last quiz and you're still doing fine in class, right?" Build evidence that one test doesn't define her. If the anxiety is really intense or affecting her sleep regularly, mention it to her doctor or school counselor. They might have other strategies or there could be something else going on.