Based on the photo and history, this looks like a mucocele, which is a common and harmless swelling caused by blockage or trauma to a minor salivary gland.
It often appears suddenly, may fluctuate in size, and is not dangerous.
Many mucoceles resolve on their own within 1–2 weeks. Avoid biting or irritating the area.
Next Steps
If it does not reduce, keeps recurring, or causes discomfort, a simple minor procedure can remove it safely.
Please don’t worry—this condition is treatable.