I think you should definitely see your doctor
! There are many potential causes of these symptoms.
One possibility is that you have an inflammation of the stomach lining, called gastritis. This can occur either from chemical irritation or after infection with a bacteria called H. pylori. It is relatively easy to treat.
Another possibility would be celiac disease, which is a common and underdiagnosed allergy to wheat gluten and similar proteins in related grains. It leads to inflammation of the small intestine, weight loss, and sometimes rashes and abdominal pain or nausea.
Similarly, inflammatory bowel disease is a condition in which various parts of the intestine become inflamed, leading to abdominal pain, weight loss, blood in the bowel movements, and other symptoms.
It would also be important to disclose to your doctor if you have any symptoms consistent with food avoidance, poor body image, or an eating disorder
. Eating disorders are also very common in people your age, and they are underdiagnosed and a very serious source of long term health problems and even death.
Start by talking as soon as possible with your primary care doctor
. They will help you decide what the next steps are in working up your condition.