Hi, I Am A 16 Year Old Girl And At

hi, i am a 16 year old girl and at the front of my mouth, my gums are startig to turn purple ( and are a little bit sore, have only just seen/noticed this today ) about a month or two ago i went to see my dentist ( my gums were not purple at this time ) and she said i had grade 4/5 gum disease, she did a full clean of my teeth and i am due for a full clean again in a few weeks or so. Why is it that at the front of my mouth my gums/lip is turining purple? i have looked up things online but yeaa why? it isnt purple right under my teeth, right under my teeth is pimk but its at the bottom of my gum and my lower lip. it is sore when touched