The accurate translation can be formed by keeping the following things in mind:

(1) Inflammation and swelling are 02 different things. In Punjabi, closest word to 'infllammation' is 'ਜਲੂਣ' (redness + swelling) which is what you feel when something like honey-bee bites your skin. In comparison, word 'swelling' just means ਸੋਜਸ਼ without the 'redness'.
(2) The word ਮਿਹਦਾ is preferred because sometimes people don't distinguish between 'stomach' (ਪੇਟ) & 'abdomen' (ਢਿੱਡ).

(3) Stomatitis is a combination of 'stomach' (ਮਿਹਦਾ) + 'itis' (ਜਲੂਣ) which can be easily translated as ਮਿਹਦੇ ਦੀ ਜਲੂਣ