My son is four (4) years old, at his wellness visit in March we talked with our pediatrician regarding some issues he was having - seeming off-balance, running into things, meltdowns in certain situations, sensitivity to tags and certain textures, hot & cold, etc. He frequently gets overwhelmed in "busy" environments and seems to hear and see EVERYTHING going on. He also has a 'personal space' issue - except for immediate family and close friends, he will completely come unhinged when someone gets too close. After asking us a few more questions, she recommended we get him screened by Emerge - A Child's Place. We set it up and he has been diagnosed with Sensory Processing Disorder which has many forms and symptoms but basically means he is "out-of-sync" between the stimuli coming in and how his neuro-net processes the information. His reactions are biological not behavior and he can learn how to simulate being in sync. We are lucky that we have a pediatrician who knew the signs and a place like Emerge right here in Durham/Chapel Hill. He is in Occupational Therapy, attending once a week to learn the skills he needs to deal with the way his system perceives the world.
Since you are concerned enough to seek out advice, I would recommend starting with your pediatrician - give him/her all the specifics you can think of with regards to your son's behavior. It may just be a phase that he is going through but if it is something like SPD, the earlier the intervention the better for the child - just knowing what is going on has made such a difference in how everyone is interacting with my son and the world is not as scary for him as it was just two months ago.
Good luck,
D. L
Durham, NC