On this kick serve, with out reading above is this a Twist your talking about or a Topspin serve.
Also, what kind of motion do you use. Both hands go up as in Roddicks style and Rafters style? Or The classic racquet goes down - like Sampras???
Rafter and many other players incurred shoulder problems with kick type serves. Studies revealed that Rafter and others were not opening their shoulders enough. Taking pressure off a nerve in the shoulder. I am sorry but I am not that "up" on what the name of the nerve was.
Anyway, the report said had Rafter learned to open the shoulders more he might have had a much longer career in tennis. Is the report accurate? I dont know but it is food for thought.
There is know doubt that the kick serve places tremendous pressure on the shoulder and lower back - especially if the kick serve your talking about is a twist serve. You really have to work on strengthening the shoulder area and your stomach to support and offset that pressure.
The soreness your talking about is it pain? Or is it like muscle soreness? Because you might want to check that out with a Doctor before serving more kick serves.
Without me typing more there are plenty of websites that tell you where to toss. Here is a good one http://www.operationdoubles.com/Serving_With_Spin.htm
This site does not show the tossing area for the Twist. It is further over the left shoulder and above the head as you hit as compared to topspin.
If you think your tossing out in front, it could be that as you toss in front you are moving forward before you hit the ball which causes you to go back under the ball, your actually hit could be too far behind you placing strain on the shoulder.
Again, without seeing your motion it is difficult to give you advice, take some of these tips and see if something is happening. Go see a Doctor!