I have dark red spots on the bottom of my mouth under the tongue on the floor of my mouth. not painful?

Don't wait: The dark spots are most likely blood vessels called varicosities. If spots weren't there before, but now they have been there for a week or so, and not going away, then please see a physician or dentist. The floor of the mouth is a really common area for oral cancer and it is best to definitively determine what the spots are than to wait for something unfortunate.

Blood clots?: Sometimes we accidentally burst a small blood vessel while eating that will cause small blood clots for form. It could also be varicosities or benign blood growths. It should be looked at by a dentist or oral surgeon to determine exactly what they are.