What Causes Sudden Appearance Of Dark Patches On Hands?

Hi, I'm a 30 year old female, 5'8" tall and weigh 160 lbs. I am in good health and have never had any major ailments or health problems. I have a large dark patch of skin on the back of my hand. It appeared overnight about 3-4 weeks ago. Today I noticed some small dark patches have appeared on the other hand? What could these be and should I see a doctor about them? Thank you for you time!