The cause of ketonuria is the body's need for energy. The fat is broken down because there is not enough insulin or sugar available to use sugar as the main source of energy. Sometimes, such as with an infection, the body needs extra energy and the fat breaks down. As the ketones build up following the fat breakdown, ketoacidosis eventually results. The most frequent symptoms are a stomachache and, eventually, vomiting.
Deep breathing is a late sign.
There are at least three parts to the answer to this question. If left untreated, death will eventually follow. The second reason for not feeling great with ketones is that potassium and sodium, important body salts, are lost with every molecule of ketone going out in the urine. If potassium is depleted, the mobility decreases or stops. Then an upset stomach and vomiting occur.