Hi ,
From the symptoms you have narrated,I think it's a cyst that commonly arises from minor salivary glands of lower lip(mucocoele). It's quite common. Treatment involves a minor surgical procedure to remove the cyst. It will be done under local anesthesia. So it is a painless procedure and you will recover very fast. So don't worry.
Next Steps
Visit a nearby dental surgeon and get treated. Best regards