Hello. About A Few Days Ago, I Noticed I Had

Hello. About a few days ago, I noticed I had a cut on my gums behind my molars. I though it would go away with time and the reason it is persisting is because of brushing and eating. I have been having tooth aches so I looked at it today and it looks almost like a rash and there is a lot of plaque on my molars. I am not sexually active. I am wondering if I should be worried.