T.R.
You an look it up on webmd.com if it keeps troubling you or if you can't get through that medical jargon and still are nervous, check with your doctor to rule out any other possibly damaging conditions. Ignoring something out of the ordinary could be cause for concern, it could be juvenile epilepsy or just a reflex. Watch to see if it happens at any other time, too. If it only happens when your changing him then its probably just a normal reflex, but if you notice it throughout the day have him check out.....You know as the old saying goes, "It is better to be safe then sorry."