I am 14 years old and for the past 6 months I've had a 3cm red blotch on the side of my jaw. After doing exercise, it reddens even more however some days it is not very noticeable. I have already been to a doctor and hospital and they have said that there is no inflammation or salivary gland infection after doing X-rays and Ultrasounds. Its getting to the point where I am self-conscious of it so I would really appreciate if someone knew what it was.

There is absolutely no way we could diagnose what it is over the internet. Have you tried a dermatologist ? The problem does not necessarily have to be internal, but topical.