The fever in and of itself is not a problem. It signals that her immune system is working. On something. You just don't know what it is. I seldom medicate unless the fever is above 100.5-101. I let the body do it's job. Although the tylenol and motrin help a person feel better and do relieve fever, why would I lower it if that is the bodies way of fighting off germs?
Several good responses are listed below. Urinary tract infection, teething with molars, ear infections, sore throats, sinus infections. I would seriously doubt any type of pancreatitis or gall bladder in one so young, but I am sure it has been known to happen. Same for appendicitis. I would think that the symptoms would be a little less vague. Blood work would tell you if the white count were elevated, therefore showing you an infection, but I would think that you already know that because she is running a fever.
Are you sure that she has no complaints? No sore ears, sore throat, pain or burning on urination? No diarrhea? No swollen lymph nodes or knots under the jawline below the ears?
If she only has a vague stomach complaint and is taking fluids and eating, I don't know what the doctor will do. He might put her on antibiotics, but it sounds like it may be a virus. He may not know anymore than you. By the time you get there she will have already weathered the worst of it. If it were me, and she were eating, drinking, no real complaints, mild fevers, etc. I would leave her at home and watch her. But we don't usually run right to the doctor at the first sign of a fever. I usually manage them at home and try everything else first before going to pediatrician. Usually, I seldom even take a temperature, even when going to the doctor. I know bad me. And I am a nurse even. So maybe I feel a little more comfortable waiting at home. My middle son had a 103.1 fever one night, sounded stopped up and had a mild complaint of sore throat. I did give him the motrin because he also said he had a headache. Gave him fluids and sent him to bed. Waited and watched. He still felt bad, so we went to the late clinic at our pedi office at about 7pm, almost 24hrs after first symptoms. Still had a fever, taking liquids ok, lymph nodes swollen, sore throat. So we went and got on some antibiotics. Now, the only reason we didn't ride this out is because in May he had strep throat twice, and was on two rounds of antibiotics and gave it to my two other sons, a friends 3yr old, and my friend. All, I suspect accidentally, drinking from the same cup or straw adn passing from one person to another unknowingly. I use lysol room spray for doorknobs, bleach in the bathroom and dishwasher, and change toothbrushes, toothpaste tubes etc, with illness, colds---especially strep throat. So go figure.
Anyway, I would take her to the doctor if you are feeling uncomfortable taking care of her or are feeling guilty for not seeking medical help. Or want a script for antibiotics. Other than that the doctor may not be able to tell you exactly what it is beyond what you have described.
Give her some yougurt with good cultures also.