I've had my RPC for 12 months and recently started flying again after being offline due to major surgery. 1st time was with previous Instructor to make sure I remembered everything, and all went well. Did 2nd circuits a few weeks later, then did my 3rd 2 weeks ago. As I landed full stop and hit the ground, for some reason I pulled the brake full immediately, not gradually. The plane skidded beyond any controlling for about 70m into the long grass (the skid marks were somewhat like a "candy cane" shape for visual). Thankfully no damage as the long grass caught the wheels and stopped the plane before reaching any of the wire fencing a few metres away. If I had been on the other runway, I would have most certainly crashed into the parked planes which were in the same side I stopped but on the other runway (phew). My instructor got a bit annoyed (his plane) but saw there was no damage. I was so embarrassed. I also hurt my left chest as my instinct during the skid was to pull harder on the brake, so I reached across with my left hand to help my right hand pull the brake in the centre console - and with a seatbelt on I strained my chest with that reach across. I still don't know why I pulled the brake so hard, it was such a smooth, no crosswind, early runway landing. It's like driving a car, you don't go from 100km/hr and pull the handbrakes. Silly me.