Dentist accidental cut under tongue during treatment. Formed puss bubble. I popped it with needle. Came back after few days. What is it?

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.
Bubble. Im sorry that happened, it does happen sometimes though we try to avoid it. Are you sure it was pus?. Sometimes it can be the the duct of minor salivary gland that got transected/cut, and in that case, you need the minor gland removed. 1) don't pop anything in your mouth with a needle. may create a worse problem 2) go to your dentist when its swollen and show them...