The only way to really get an accurate diagnosis and specific treatment plan is to be examined by a qualified hand specialist who cares of a lot of throwing athletes.
But it does sound like you have De Quervain's tendsynovitis.
If so, doing this motion will likely cause you pain:
You may want to read more about it on the Mayo Clinic website: http://www.mayoclinic.com/health/de-...SECTION=causes
Here is their description of its cause [with some added comments by me in brackets]:
"When you grip, grasp, clench, pinch or wring anything in your hand, you use two major tendons in your wrist and lower thumb
. These tendons run side by side from your forearm through the thumb side of your wrist. They normally glide unhampered through the small tunnel
that connects them to the base of the thumb. In de Quervain's tenosynovitis
, the tendons' slippery covering [called the synovial sheath
] becomes inflamed, restricting movement of the tendons.
Chronic overuse of your wrist
is commonly associated with de Quervain's tenosynovitis. For example, wringing out a cloth involves a repetitive motion, a bent wrist and the gripping of the cloth. [Similarly, ''laying the wrist back" to "aim the butt of the racquet at the ball", then rapid ulnar extension of the wrist as you snap the racquet forward as you go to hit the ball on groundstrokes or the serve, will involve a repetitive motion that moves these two tendons through the narrow tunnel at the wrist.] If you repeat an action like this day after day, this combination may be enough to irritate the sheath around the two tendons. [That synovial sheath should normally be very smooth, and even contain a "slippery fluid" to let the tendons slide, but with enough overuse, it can eventually become inflammed.]"
Rest from tennis is required to let the inflammation subside.
If you keep "playing through the pain" you risk the inflammation becoming more advanced, and the delicate sheath that should provide a slippery tunnel can get all scarred up, and never be normal again.
Note that other motions, including lots of movement of a computer mouse, may also exacerbate the symptoms.
Sometimes a cortisone injection is necessary to stop the inflammation.
Sometimes a splint is necessary to prevent any sliding of the tendons, even while sleeping.
Sometimes, the problem keeps recurring, and the tunnel and/or synovial sheath need to be openened in a surgical procedure to decompress the tendons.
But really, do yourself a favor and get examined by a local expert who can make a definitive diagnosis, and guide you through until you are "all better", and even help design a physical therapy regimen to help prevent recurrence.
Best wishes for a speedy recovery.