This is Typical case of Blood Blister. Blood blisters are most commonly caused by oral trauma and cheek-biting. Blood blisters form when blood vessels underneath the epithelial mouth tissues are ruptured, such as when one accidentally bites the cheek tissue while eating or speaking. Blood escaping these vessels can pool up within the mouth tissue, which forms a blister.

Hello Good morning, It's cheek bite and blood clot ... Nothing to worry it will heal by it self.... Visit dentist if its reoccurring to round off the sharp cusps of your teeth