Autism / Autism Spectrum Support Group
Autism is classified as a neurodevelopmental disorder which manifests itself in markedly abnormal social interaction, communication ability, patterns of interests, and patterns of behavior. Although the specific etiology of autism is unknown, many researchers suspect that autism results from genetically mediated vulnerabilities to environmental triggers.
I'm not a doctor or anything, just a person with ASD myself, so this is my guess. So even if my advice is useless, I hope your child gets back on track with the PT (my girl with Down Syndrome also regressed - so frustrating). Hope everything is alright.