Hi king1234. Doxy can be hard on the stomach, especially if you don't know the tips. Do you have nausea from it? It helpls to eat something substantial before taking Doxy. What helped me was to have a sandwich with some kind of protein. Do not have any dairy, iron, or magnesium products within two hours of taking it.
Make sure you drink a full glass of water when you take it or it can ulcerate your esophagus. Do not lay down for at least an hour after taking it for the same reason.
I hope these things help. You should let your doctor know about your symptoms.