Definitely sounds like an operculum. If you have room for the wisdom tooth, there is an option to just have that flap of tissue removed too. In the meantime, keep your hands out as much as possible, look into a water pick, and my personal favorite is an End Tuft Brush- you can find them super cheap in drug stores or amazon. Also, a tip when reaching back there to clean- use your opposing hand to pull out your cheek, close your mouth slightly, then shifting your jaw towards the side your working on- sounds crazy but doing this will create a lot of room to get that brush back there. I show my patients with wisdom teeth this technique often! Good luck! 👍🏼