Being that it was your first practice of sanchin, it is perfectly normal that you became sore. It is the same way when you begin weight training.
Soreness, however, is not a direct indication of an effective training program; i.e. being sore doesn't mean it's working, not being sore doesn't mean it's not working.
Once your body adapts to sanchin, the soreness will decrease, as the intensity of your training increases. If the soreness from sanchin continues for many weeks, reduce the intensity a bit, and ease into it.