There is many causes of red tongue most common among them is stains caused by any red coloured food which we eat like Strawberry etc , but this is temporarily. As your lesion is since 3-4 months it can be Vitamin defficiency like Kawasaki disease or streptococcus infection like scarlet fever. Now more serious cause can be Erythroplakia which is red area or lesion that cannot be rubbed off .A lesion with combination of red and white appearance is called as Erythroleukoplakia.These lesion are all considered to have premalignant potential. Erythroplakia and Erythroleukoplakia have an increased risk of premalignancy. In addition of appearance cause of concern can be if lesion does not go away or it is growing in size.