I've got a lump/flap of skin inside my mouth where my cheek is near my tooth?

Wisdom tooth. This flap of tissue is common around erupting wisdom teeth. It is easily traumatized and difficult to clean around this so food debris can get caught under this flap and infections can occur. See a dentist and get this checked.
Lump/flap? This could be so many things: related to a wisdom tooth, Parotid duct where salivary fluids enter the oral cavity, linea alba from cheek biting, mucocele or a growth. See a dentist who can personally examine you, give you an exact diagnosis and suggest treatment, if needed. We cannot diagnose these types of situations over the internet, sight unseen.
Stensen's Duct. If limp is on the cheek, opposite the upper 6 year molar (the 6th tooth back), it's the duct where the Parotid Salivary Gland empties into the mouth. If it's anywhere else, make an appointment with your Dentist.
Wisdom tooth. Likely a wisdom tooth if in the back. If so, see the wisdom tooth expert. For best results with wisdom teeth, make sure you are evaluated and treated by a board certified oral & maxillofacial surgeon.