Hi,

Thanks for asking the query,

The hard lump in your mouth can be due to infection drained from any infected tooth, other then this this could be an allergic reaction, cyst or tumor.

A thorough clinical evaluation of the region is required.
Take an OPG radiograph.
If there is presence of any infected tooth, drainage of the abscess and root canal treatment of the tooth is required.
Also get a checkup done by an ENT specialist.
Start with the course of antibiotics and analgesics.
Take lukewarm saline and antiseptic mouthwash rinses.
Maintain a good oral hygiene.

Hope you find this as helpful,
Regards.

There are multiple possibilities. You may have what is call "Torus palatinus". That is a fancy word for a bone grow on the roof of your mouth. Tori (plural) can happen anywhere in the mouth. They are idiopathic, which means we don't know why they grow. They are typically not painful, but are much more easily irritated than the rest of your mouth. Tori stretch the tissue over them out, which make it thinner and more prone to injury. A tori, can grow larger, but should not fluctuate in size. It also should not give you sinus pain.

You may have another form of cyst or grow, either related, or unrelated to your surgery. The best course of action would be to request further investigation. This might include referral to an Oral Surgeon or a Oral Pathology. They can biopsy the lesion and tell you for sure. There is also the possibility of imaging the area in 3D. It's called a Cone Beam CT (CBCT) and can give you a reasonable idea of the area.

Please continue to push your dentist. If he is unwilling to refer you, then locate and contact a specialist on your own.