A lump under the tongue, behind the teeth can be a number of things.
These are the possibilities:
1. Inclusion cyst: a harmless round mass in the mucosa of the mouth, much like a pimple in a different location. Often are not so hard and can be tender if infected.
2. A salivary duct stone: many saliva glands open in this area, and their tubing courses through the bottom of your mouth. These are usually hard. Not a problem unless the gland gets swollen.
3. A torus: This is a very common, benign growth of bone off of the mandible.
4. A vascular mass: these are often under the tongue in a blood vessel, and are usually harmless.
5. A Sialocele: this is a collection of saliva in the duct that can become enlarged and can get infected. They are usually softer and more tender.
6. An oral cancer: any mass in the mouth has the potential to be a cancer. These are often irregular, non rounded, hard, and non tender. These are most common in people who smoke or chew tobacco.
So, since this is such a new bump for you, you want to hydrate well, pay attention to it and if it persists beyond the week, you should see a doctor to make a possible diagnosis. If it remains hard, often these are surgically removed for biopsy. You may require an ENT or oral surgeon to look at this, and your primary doctor should see it first and determine what needs to be done.
I hope this was helpful.
I'm happy to answer any questions about this. Let me know if you need any further clarification.