shaken head syndrome?

I'm just paranoid and nervous now because I read an article about it 

My baby is 2.5 months old and likes to be sat up - she bobbles her head all the time! Like in the car when we are in a bumpy road her head bobbles. There's been a few times when my husband and I have been holding her and she is supporting her head and then suddenly she looses all support of her head.

Of course we try to support her head as much as possible! I'm just worried of long term effects this might of had on her - especially in the car! She is fine and shows ansoliitly no signs of problems at all but I read in this article that symptoms such as learning disabilities and even meningitis might not occur til later! To cause this does it have to be violent shaking?