What does painful and swollen gums indicate?

Hello doctor,

I am an 18-year-old, who dislikes visiting the dentist. Therefore, you can imagine my hygiene is good. I had a brief period of gingivitis as a result of using braces for three years. Having removed my braces over a year ago, and lucky for me, I do not have wisdom teeth, the end of my dental pain seemed over. However, about five days ago all of a sudden, I woke up and the very back of my mouth around the last left molar is swollen. It is sore and irritable. I kept brushing and using salt water. There was no bleeding or pus, but it started to feel more irritable and tender as if it was an infection. It has not gotten worse in swelling or pain, but it has not gotten better either. My mother predicts a piece of food got wedged in the gum line, near my tongue. We have tried using gel to numb it enough so I could use floss or a toothpick to work around the gum, but it still is very painful to touch and it is very hard to reach that area.

Sadly for me, I am starting my Invisalign treatment and I am going to get the impressions tomorrow. And I also have an appointment for regular cleaning. I am petrified. Do you think a piece of food is the problem? Is there something I can do to remove the food before my appointment tomorrow? What will the dentist do tomorrow when I ask him to address the situation? What are the chances this is a bigger infection and is not food-related, what will the dentist do then? Will I have to have surgery or a procedure done or antibiotics will help?

Please suggest.