It could be just a cavity from what you have described. Please visit the dentist as scheduled. Let the dentist decide what will be the best treatment . Please don't be scared about root canal treatment. Dentistry has advanced so much that now lot of treatment can be done fast painless by the specialists .
Next Steps
get the necessary treatmen done from your dentist soon. don't be scared and don't waste time. cause if the problem can be solved by just a filling that same tooth may require root canal treatmen if you prolong time .