What Causes Bruise Under The Chin Near The Jaw Line?

My child is 13 years old female. She had an unexplained bruise located under her chin near the jaw line. She didnt remember falling or injury. A month later, the bruising return again last night for no reason. The bruise starts right under the bottom lip in a straight line to larger bruises under the chin at the jaw line. My doctor does not seem concerned, but I am a little. Should I be worried?