Have a bright red mark under my tongue, looks like a broken blood vessel. What would cause this? I am a healthy 50yr old female.

See your dentist. Could be a broken vessel or hematoma from injury (toothbrush, food, etc.), but tough to tell without a photo. There are many blood vessels (varices) under the tongue-some can appear large and superficial--hence the large purplish-red area. Could be a benign lesion of blood vessels (hemangioma) or a "port-wine stain". Lack of pain does not equal safe. Get it checked out by your dentist to be sure.