Hi.. Welcome to HEALTHCARE MAGIC.. I have gone through your query and can understand your concerns.. As per your complain it seems that popping up of a bubble over the gum that released sour sweet stuff anion..d leaving a red indent at the place seems to be most probably a gum boil or a gum abscess that can be due to infection in the gums in that area leading to accumulation of bacteria and causing pus formation.. As smokers are more susceptible to gum infections which can become severe due to the cause that in smokers the symptoms become visible once the gum disease progresses and not visible initially.. As far as the abscess is concerned you should consult an Oral Physician and get evaluated and a deep scaling followed by topical application of gum paint and antiseptic mouthwash gargles along with warm saline gargles will help.. Hope this information helps.. Thanks and regards. Dr.Honey Nandwani Arora.