It might have gotten dissolved...
Scenario 1: You probably passed the stone during the week, which elapsed between the 2 X-rays, which would be good news, specially if you are no longer in pain.
Scenario 2: If you are still in pain, you may still have the kidney stone, although there might be other shadows obscuring the right kidney on X-ray that prevent the radiologist from seeing the stone on X-ray. If you are still in pain, you might need a CT of the abdomen and pelvis without intravenous contrast (the ultimate test) to determine whether you have a stone in your right kidney or not. This is the best test to check whether you have a stone or not.