head jerking

hi, i was wondering if head bobbing/jerking can be associated with autism..my 3 year old daughter is autistic and i have noticed lately that at times bobbs her head up and down...yesterday i was fixing her hair and in the middle of doin it her head started to bobb up kinda of like when someone sees a friend in the streets and bobbs their head head up to say hi. its hard to explain...i also has another autistic daughter and she does not do this but my 5 year old daughter does this as well at times....im not sure what to do..any help please n thanks!