I have been a dental hygienist for 12 years. It sounds to me like what you are seeing and feeling on your molar is a build up of tartar. It can get really thick, make the tooth look larger and feels like sandpaper. It typically doesn't build up much on the biting surface because chewing prevents it but can get very heavy on the sides.. You might feel pain when touching it due to gum disease or an abscess. If you are unable to brush and floss regularly like you mentioned it makes sense that you would get an excess amount of tartar and the upper molars are a spot that most people get heavier build up even with regular brushing.