I'm by no means a teacher but... It's normal to feel some tension and even muscle soreness when practicing new pieces, chords or scales. Especially when you haven't played for long. In fact this is an evil necessary, since your muscles needs to grow and get stronger, just like with any other training.
But you should stop immediately if you feel pain, this is most important. And be sure to take frequent breaks.
All the best from Sweden.
1965 - Masaru Kono No.5
1975 - Atushi Nakamura No.15
1977 - Kuniharu Nobe No.15
1987 - Curt Claus Voigt
1996 - Masaru Kohno Maestro
Feel free to ask me anything about Japanese classical guitars.