What Causes A White Flap Of Skin Under Tongue?

My 17 yr old non sexually active daughter has a white 3-4 mm flap of skin under her tongue. It does not hurt her, but it bothers her that it is there because anything in the mouth feels huge, and she messes with it with her tongue.No pain, just bother. She is guilty of eating an entire bag of sunflower seeds a couple of weeks ago. Any ideas? Worried mom The only problems she has had were bleeding ulcers in her stomach at age 2 and 1/2, almost died, had to have several blood transfusions, Tagamet did the job. Irregular menses. weight 140 steady and 5'11" height.