Brief Answer:
Follow the advices

Detailed Answer:

Hello,

Well, it could be due to several causes like any injury to the tongue (due to consumption of very hot food/drink), some trauma during brushing, dental caries, infection, nutritional deficiencies or even due to an impending cold/flu, identifying the culprit will be helpful as treatment can be done accordingly.

Meanwhile, you can start doing gargling with chlorhexidine mouth-wash twice daily and gargling with luke-warm salt water twice daily in an alternative fashion. Use a soft bristle toothbrush.

Avoid eating spicy/fast foods, have fresh home-cooked meals and plenty of fresh fruits, salads; drink plenty of water throughout the day. Avoid cold-drinks for the time being. Strictly avoid smoking if at all do.

Along with this, I'd also suggest you get a Dental check-up done to rule out any underlying problems.

Take Care
Kind Regards

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