Paul sez: here's some comments I e-mailed a young violinist who had
buzzing and numbness in her left hand and arm.

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I'd be glad to give you some comments.  I very recently had to undergo
physical therapy when an increase in my playing time gave me problems
somewhat similar to yours.  Also, I have a 13 year old daughter who
also plays violin.

First off, an increase in practice or playing time is  a very common
precipitator
of these kind of problems.  If you've gone from an hour a day to three
hours, it is not trivial to your body, particularly when it involves
the kind of odd postures violin demands.  I am glad to hear that you have
increased your time by doing several shorter practices instead of 
lots of unbroken time.  But I think you are going to want to back off
on your practice time until you get the problem under control.  In
these kind of injuries, the first stage is where you get pain or
problems WHEN you are playing.  The second stage is when you have the
pain *even when you are not playing.*  So that indicates things need
to be taken more seriously.  All of your skill and potential will
go for naught if you try to push through the pain, so don't let the
fear that you aren't practicing enough push you to hurt yourself
and mess things up for the future. 

You are pretty astute in suspecting your shoulder area.  The main
nerves that supply your arm and hand come out of the spine in your
neck and run through your shoulder in an interconnected web called
the brachial plexus.  Pressure on the shoulder, tight muscles in
the neck/shoulder/upper back, and other things that impact the
brachial plexus will indeed be felt as buzzing or numbness down
in your arm and hand.  In my own case, the problem was really in
my BACK and shoulder blades, which were weak and had lots of
"trigger points", which are sensitive, painful knots in the muscles.
The doctor said these muscles needed strengthening, and the physical
therapist gave me exercises and stretches for that, but also 
did massage on those trigger points and did nerve stretches.  It
took a while but the problem did clear up.

I have an unsubstantiated guess that besides what the doctor said
was wrong, that part of the trouble may also have been  1) pressure
on my shoulder from my guitar strap and  2) a 30 year habit of
holding my shoulder up when I play guitar.  (Note that my problem
was only on the left side.)  I see a parallel to violin, where you
do have a shoulder rest, and do have to hold up your left arm in
a somewhat unnatural position.  (I was increasing my violin playing
at the same time too, so that may have contributed.) As I watch
mobs of older Suzuki violinists in recitals, I always am struck
at the great variation in how they anchor their violins: many of
them really crank their heads over and are clamping that violin
with their neck muscles.  Others have basically erect heads and
violins apparently effortlessly lying on their shoulders.  Again,
anything that is either putting pressure on the neck and shoulder
or causing a lot of tension there is a potential source of trouble.

I'm no doctor, and you should see one.  If your family questions the
need, it should be a reasonable argument that if you are having pain
or numbness even when you are not playing, every authority on 
cumulative trauma injuries says that this indicates a problem that is
more than the usual "I did too much today and as a result I'm sore."
And as my own case points out: it may not be the "fault" of something
you are doing but may be connected with some particular aspect
of your own individual body.  

But also, along with any medical care or therapy you might eventually
recieve, it's important to re-examine how you use your body.  Are
you taking lessons from a teacher?  If so, what do they say?  If not,
you might seek one out and explain your trouble.  And whether you
take lessons or not, it might be a brainstorm to find a different 
teacher - perhaps somebody on string faculty at a nearby college - 
and ask them, not for ongoing lessons, but just for some of their
time to have them critique your posture, playing habits, etc.  You may
learn something that you and/or your current teacher did not notice
before.

Along those same lines, the books by Kato Havas or Yehudi Menuhin
listed on my Web page are worth seeking out in the library.  But
getting advice in person from a sharp teacher is best, I believe.

Besides actual technique instruction from books or a teacher, 
taking up some sort of physical discipline that contributes
to your body self-awareness is something I always recommend.  I
did some reading of books on Feldenkrais Method and Alexander
Technique; got a Feldenkrais tape; learned a lot of stretches,
many of them taken from yoga; and eventually took Tai Chi classes
at the local YMCA.  All of these helped me learn to use my
body with minimal tension and greater awareness of what is the
most effective and least stressful way to do things.  If you
are not involved in any physical fitness activity at all
(even walking for health) , think about doing so.

Got to run right now: I hope this helps.  I know a couple of local
teenagers with exceptional string ability who got hammered by
tendinitis and had to fight their way back.  You have a better
chance with this because you are young and have taken notice
of it early on.  Let me know any other questions you have and
how it goes with you.

Blessings!