Can Being Hit In Back Of Head Cause Headache After 10 Days?

a friend of mine was in a rear end accident over a week ago and the only thing she says hurts at this point is a headache that she says wont go away...could that still be a lingering effect from the whiplash she suffered 10 days ago or would that be something unrelated? she was hit from behind while she was at a complete stop at a light and it was estimated the guy who hit her was going around 10-15 mph...I was just curious because it always seems like when something happens to her and she does become honestly hurt, its always the most extreme case...thank you for your help