What Causes Small Brown Spots On The Hands?

I notice that brown tiny spots which resemble sun spots appear on the back of my hands overnight every so often. I have been noticing they appear frequently after a very stressful day or situation. They appear overnight, but it takes about a month or 2 for the spots to disappear completely. I m a healthy 45 year old Hispanic woman, don t smoke or drink, healthy diet and active. I do drink one cup of Cuban coffee daily with milk, I m not taking any medications for anything. No drugs. Very healthy, thank God. No known allergies. This is not caused by purpura. No broken capillaries. Very smooth skin except for the sudden brown spots. They don t itch or do anything. They just show up suddenly and slowly disappear. Would like to know the cause or probable diagnosis.