I do not have a child with autism but have spent time with one child who is autistic and one of my co-workers had asberger's syndrome which is in the autism spectrum.
I think that your concerns are real and that while mothers often over-worry, we also have great intuition that we need to listen to more.
My guess is that you have already been online doing your research and looking at the information that is available. If you haven't already looked at
Autism Speaks, that is the first place I would go to learn about the signs of autism.
I think it's important to remember that autism is a spectrum disorder which means children can have a mild form of it or even a debilitating form. Also keep in mind that the signs of autism can develop or worsen at any time. So, a child that was behaviorally "normal" for 12, 18 or even 24 months could one day start showing signs. The statistics for autism in the United States show that
1 in 150 children is diagnosed with autism. Here is a good
wiki article about autism.
The sign that concerns me is you described your daughter hitting her head on the couch while watching television. If this only happens once in awhile it might be just an odd thing she likes to do. But, if she often engages in behavior like this and at different times (not just in front of the TV)--a repetitive motion--like shaking her hands or hitting her head--in the "autism world" this is called "stimming" or "self-stimulation" and it is strongly associated with autism.
Having said that, when I was an infant and in the process of learning to talk (I was an early talker, can you tell?:rofl

my mother says that I would get so frustrated with not being able to express myself so the adults in my life could understand that I would do things like hit my head on the floor in frustration. So, odd things like that can have other explanations. No, I do not have autism and eventually I learned to talk and stopped hitting my head on the floor.
But, you are right to seek the assessment of a doctor. I think that others may be able to provide you with specific information about doctors in Hong Kong that have experience with diagnosing autismbut contacting
Autism Speaks and asking them for references is not a bad idea either.
If my child was having any of the difficulties you've mentioned I would definitely take him to the doctor to be assessed. If your child does have some problems, at least you can know how to help her. And in the best-case scenario (which a big :gl::gl::gl::gl: to you!!!) is that the doctor will just say "She's just this type of girl and very shy and this is how you can help her..." I really hope that that is what the doctor says! Again, :gl:!!!