one online definition: "Ketones: Chemicals that the body makes when there is not enough insulin in the blood and it must break down fat for its energy. Ketone bodies can poison and even kill body cells. When the body does not have the help of insulin, the ketones build up in the blood and then "spill" over into the urine so that the body can get rid of them. The body can also rid itself of one type of ketone, called acetone, through the lungs. This gives the breath a fruity odor. Ketones that build up in the body for a long time lead to serious illness and coma."
the whole low-carb thing is designed to specifically put the body into low level ketosis to burn fat.
if you didn't have many carbs for lunch, and then walked around in the heat, that could definitely have caused a ketone spill... but if it's a one-time thing, it's not a big deal at all. it's just your body finding enough energy to keep you going when you've used up the immediate available food sources.
if you're not feeling dizzy or ill, you're probably just fine. a little ketosis isn't really a big deal... it's only when it's been going on for a long time, and in large amounts, that you start looking at problems.
hope that helps!