When Romeo is wounded on the beach by Tybolt, his wound is on the left side. However, when Romeo goes to Juliet, the bandaged wound is on the right side.
I own the movie on video and when he goes to see Juliet his wound is
still on the left side (the side away from the camera) I think you should watch the movie again!
At the beach, his wound does appear to be on his left side, which it is. You are seeing Leo from the back, so his wound is on the left. When you see him from the front, at Juliet's house, his wound is still on his left, but you are seeing it from a different perspective.