Yesterday, I got hit on the head with a tennis ball. The ball hit mildly hard, on the right side of my head. It did not hurt. Today, it hurts when I do rigorous exercises. When I did some jumping jacks, the right side of my head hurt and it gave me a mild headache. When I go down the stairs, the right side of my head also hurts mildly. Is this just a short symptom or something serious?