Either sun burn or I suspect infection. Treat with Physan or an anti-fungal ASAP. Also, ensure plenty of air movement around this plant. Check too to make sure there is not any direct sunlight hitting this plant. One summer, after a hail-storm in the spring, I notices lots of weird burns happening to my orchids. The hail had caused small openings in the roof shade-cloth and direct light would come through those openings and burn leaves.