The lymph nodes in the armpits are the first line of defense when pathogens invade, which also means that your immune system is in good shape and you don't need any treatment, you'll be fine on your own. Many people will take antibiotics, antiviral drugs, which are actually inappropriate. Enhanced nutrition and vitamins, hot baths, and drinking lots of water are actually more appropriate.

I speculate that it is due to the fact that you caught a cold last night, or it may be that the small yellowtail is not fresh (there are many negative Vibrio bacteria in seafood, but if there is a problem, it should also be a tummy ache), and the smallest chance of it being an allergy is probably the least likely one, that one is very complicated, and can't be explained in a few words.