Teenager with Gastro Problems

My daughter is 18, and has had problems with her stomach for years. She is frequently nauseated after she eats, and has numerous bowel movements every day. She is intolerant now of sugar, chocolate, and dairy products. The problem is getting worse, and she continues to lose weight. Is a gastrologist the best bet to diagnose the problem or should she see an internal specialist first? She has taken medications that the doctor has perscribed (general practitioner), but it seems to either treat symptoms or makes her more nauseated. Perplexed....