hello and welcome,
After reading your history i can suspect you hard palate
is inflammed and has developed swelling.
because of the swelling , hard palate appears smaller or shallower.
Inflammation can be due injury to the palatal mucosa , mechanical or chemical injury , allergies due to food products , sore food stuffs , hot or spicy foods .
INflammation of minor salivary gland
may lead to palatal swelling.
Ulceration caused due to viral infections like herpes.
leading to bacterial or fungal infections of the palatal mucosa.
exact diagnosis can be made only by clinical examination so get it evaluated by a dentist
Meantime you can use antiseptic mouth wash like betadine or chlorhexidine
for oral rinsing.
apply dentogel ointment over the palatal mucosa will give relief.
i hope this helps,