Hi, from what I understand, you are talking about your 3rd molar (wisdom tooth). If I am right then I would like to tell you that 3rd molars are prone to getting decayed and stains since its the last tooth in the arch, so regular brushing techniques if not followed properly, tends to accumulate stains and plaque. I would recommended you to consult a dentist
to get your tooth clinically evaluated. Regards Dr. Dhruv anand.