Hello and thank you for writing to JustAsk!
 
As a School Psychologist who works a lot with children under five years of age and their parents I often get this type of question.
 
 I know that Autism is in the news and unfortunately new statistics show that as many as one in every 90 children may have a form of the disorder.  However, you describe your baby as attentive, laughing with you (which means interactive) and seemingly otherwise to be developing on schedule.  Thus, these are the signs of a typical infant.  Moving back and forth is often fun for a young child and one of the ways they practice motor skills prior to walking.  
 
If you have concerns you may wish to 1) discuss them with your pediatrician  2) request an early intervention evaluation from your school district or 3) refer to many of the online resources that can help you with your questions and guide you, if needed.  
 
I have listed below a few websites that may be of help to you.  Also, you are not "paranoid" but a good parent, as you were brave enough to ask your question here on our forum.  Thank you for sharing with us and allowing us to help you and also others by answering this question.
 
Good luck!