Suggest Medication For Cough,cold,congestion And Stomach Pain

My 3 yr old niece, is constantly sick. She complains with her stomach all the time. She has been to the doctor numerous times, but the problem still persists. She also is always coughing with super runny nose and congestion. They have her on allergy medication and now on nasal inhalers and breathing treatments. I just don t seem to understand why these problems persist to never go away. Her stomach concerns me because she does throw up at times and does have diaharrea.