Why have I developed a lump or a bubble overnight under my tongue?

Hi,

Welcome to icliniq.com.

I understand your concern. If the lump is big enough to be held by your finger when you palpate physically, it may need a biopsy. However, from the photo that you have shared (attachment removed to protect the patient's identity), it looks like a common ranula to me. If the sublingual salivary glands are blocked or there is some minor obstruction, then a ranula can form. I suggest you to start rinsing with warm salt water several times a day, and start using a Chlorhexidine mouthwash. Usually a ranula is self-limiting and resolves on its own. I advise you to observe for the next two days if it is reducing in size or not. I will suggest you surgical excision of ranula only if it does not resolve after rinsing and mouth washing. I suggest that you can also use a Lidocaine 2% and Benzocaine oral pain gel to that area to reduce the size.