There are many reasons for redness under the gums, such as external stimulation, gingivitis, periodontitis, periapical periodontitis, and so on. 1. External stimuli: brushing too hard, eating too hot food, etc., may lead to gum epidermal damage, so the phenomenon of redness under the gums. 2. Gingivitis: Gingivitis can also lead to redness under the gums, and even swelling and pain in the gums. 3. Periodontitis: Periodontitis can also cause redness under the gums, not only redness, swelling and pain, but also bleeding gums. 4. Apical periodontitis: Apical periodontitis can also cause redness under the gums due to inflammation. Because the inflammation breaks through from the alveolar bone, it can spread to the gums, thus showing redness under the gums. There are other causes of redness under the gums, it is recommended to go to the hospital in time after the appearance of symptoms, under the guidance of the doctor to take standardized treatment, do not ignore the symptoms, take it lightly.