Is it always autism?

NO!!! We're too quick to jump to conclusions when a child doesn't tick all the boxes for 'normal'! (Can you tell this is another of my personal bug bears?!)
Everyone has heard of autism but few know anything about receptive language difficulties or other language disorders which can mean that the child has trouble making sense of the world, fitting in with his peers and communicating his needs and thoughts. These are all symptoms of autism but do not necessarily mean the child has ASD. My own clinical case-load has several children who have been referred for ASD assessments by their school but actually have language based disorders. As they've already been referred when I pick them up it's too late to stop the parent needlessly worrying.

This is why  proper assessment for a differential diagnosis of ASD needs to be very thorough and requires Paediatrician, highly specialist speech & language therapist and expert Clinical Psychologist. At Small Talk our assesments follow the NAS guidelines. A one-off assessment in clinic is not sufficient to label a child for a life-time.
We used to have a term called semantic-pragmatic disorder: Semantic refers to the meanings of words and phrases while Pragmatic refers to the use of language in a social context (knowing what to say and when and how to say it to other people). We now try to separate the two so that we can work on the exact needs of the child. 
The Charity AFASIC have lots more information and they describe it in a way I can't improve upon. The following is from their website www.afasic.org.uk © Afasic 
"Children with semantic difficulties:

• Struggle to understand the meaning of sentences or words. 
• Words which refer to abstract concepts such as feelings (sad, puzzled, jealous) or status (important, official) can be especially hard for these children to understand. 
• They misinterpret and take literally phrases such as "Cut it out!", "Stop it" or "He's a push-over". 
• Often it can be difficult for children with a semantic disorder to work out the topic or key information in a sentence. 

Children with pragmatic difficulties struggle:

• To use language socially. 
• They do not understand how we take turns when we talk. 
• They interrupt more than is acceptable. 
• Their conversation often seems inappropriate or irrelevant. 
• They can seem to be unaware of what their conversational partner needs to know - and can therefore say too much or too little about a subject. 

Some children have both semantic and pragmatic difficulties" other children might have significant receptive language impairment which lead to problems understanding spoken language. I met a 15 year old earlier this year with severe behaviour problems... he had an auditory memory of just 2 items, no wonder he punched first, he knows what outcome is expected from that type of interchange! Imagine how frightening it would be to live in a world when you 'just don't get it' all the time (to use his words).
One of the only benefits of the long wait for ASD diagnosis in Staffordshire (up to 90 weeks for some) has been that it's now clearer when the child's problems are more language based than autistic spectrum based. The difference lies mainly in the rigidity of thinking.
I can highly recommend the AFASIC web site for any parent or professional who wants to know more.
We know more about autism as society now so its time to spread the word about other disorders and problems that affect our chidren's ability to communicate. Problems of any kind, however mild, will certainly lead to isolation, frustration and an inability to fully integrate into society. Children will have a harder time making friends and fitting into nursery or school.