My left anterior thigh muscle feels different than right. when i rub it it feels raised in the middle. negative ultrasounds before. what could it be?

Most commonly: This is just the result of the muscles of one leg being larger than the other. If a distinct mass is present then other entities such as lipoma, muscle herniating through the tissue (fascia) surrounding the muscle, blood collection (hematoma), and (rarely) malignant tumors are possible. Mri, when indicated, is better than ultrasound in evaluating possibilities.