Welcome to the forums.
A reading of 8 mmol, while high, is not alarming. For one thing, the urine strips are not particularly accurate, and their readings are affected by factors such as how hydrated you are. And if you fasted while you were sick, your ketone levels might also have risen because of that.
As long as you are eating very little carbohydrate you are going to be in ketosis. Now, it is true that Phinney and Volek characterise the range of about 1.5 to about 3.0 as “optimal,” but their definition is not based on urine ketones, but rather on blood levels of the principal ketone body, β-hydroxybutyrate (the urine strips measure acetoacetate).
Harmful levels of ketones aren’t a concern until they rise above 10.0, and the real danger doesn’t start until even higher than that. The reason for the warning on the urine strips is that they are primarily intended to be a quick and easy way for Type I diabetics to determine their risk of diabetic ketoacidosis, and the level of 10.0 is intended to give the patient time to seek treatment before becoming dysfunctional.
Many people find that their urine strips eventually stop showing much ketone production, and the reason is that the kidneys get better over time at filtering ketones out of the urine and redirecting them back into the circulation. So all in all, I wouldn’t worry too much.
BTW, you can avoid some of the flu-like symptoms resulting from a ketogenic diet by keeping your salt intake up. Your kidneys excrete sodium more readily on a low-carbohydrate diet, so we need to make sure our salt intake is higher to compensate. Ten to fifteen grams of table salt a day, including the amount already present in food, is a good range to aim for. Salt intake in the right range will also keep your calcium, magnesium, and potassium in balance, and ordinarily eliminates the need to supplement with those minerals.