Fluoxetine for anxiety: should I increase my dosage?

I am a 15 year old male, and I currently take 20 milligrams of Fluoxetine (Prozac). I have been on this dose for about two years now. Lately, my anxiety has increased. What causes my anxiety is my phobia of becoming sick and vomiting. Because of this, I have a constant stomach ache brought on by the anxiety. If I increase my dose to 40 milligrams a day (2 pills every morning), will this cause my anxiety to settle? I tried this dosage once for about a week and it just seemed to make things worse, however, I think that was just because the medicine hadn't kicked in yet.