They can grow as hard lumps when the tumors are hollow and full of fluid. It can also occur on both sides. Black Spot Inside Cheek Pictures Causes Treatment And When To Seek Help Swelling can occur on just one side swollen right or left side upper or lower part or inside the mouth depending on the root cause. . This biting is somewhat intentionally. Cheek swelling is a symptom that arises when the cheek develops a lump or becomes larger than normal. A swollen cheek could be a result of a tooth abscess allergic reaction or infection. England Mar 20 2021 1 Hi Got some new guinea pigs a week ago 6 months old and one of them has a large very solid lump on its cheek below the skin the size of a grape. Some of the possibilities are. Ulcers black spots or callus along bitten inside part In case of a chronic inner cheeks biting and ulcers a line of linea alba is presently visible as a long stretch If there is pain it could indicat