Blister bubble under tongue from cut by dentist filled up siZe of two peas and popped by itself. No pain. What is that and what should I do?

If it recurs.. Get it checked. It is most likely a mucocele which forms from interruption of salivary gland drainage ducts and if it is gone now as implied there isn't much you can do. If it recurs, especially after putting something very sour like a lemon drop in your mouth, try to get it looked at before it disappears again. Of course we can';t diagnose over the internet but if it stays away, no worries.
Hematoma. Could be a "blood blister" from cut blood vessel. If you have no symptoms, no treatment indicated. If it recurs, or you develop pain, see your dentist or an oral surgeon.
Possible Ranula. A ranula presents as a fluid filled bubble on the floor of the mouth under the tongue. It is caused by a blocked salivary gland. The saliva can't get out so it backs up and creates a bubble. Small ranulas can often resolve on their own. Larger ones (shown here) may require surgical removal. If it reoccurs have it evaluated by an oral surgeon otherwise don't worry as it has resolved on its own.
Mucocele. Usually they're not a problem and will go away on their own. It can be a common result of a constant trauma. If concerned see a dentist for evaluation and treatment if needed.