There can be a million of reasons/diseases involved here. Simply because you introduced new fish without quarantining them. They might've brought some infections or parasites with them. Or, you might have over crowded the pond (I wouldn't know since I don't know how big the pond is) meaning that the nitrates are getting dangerously high.

1. Test the water parameters: pH, ammonia, nitrite and nitrate If you don't have a test kit, buy one. Or if you can't afford your own now, ask at the pet shop, they might be nice enough to test the water for you if you bring a small cup of the pond water
2. Look at the fish, CLOSELY. Do you see any symptoms? Are they eating well? How are they acting? Are there wounds? Spots, streaks, excess slime coat, fraying fins...?

I'd suggest doing some water changes, 20-50% after taking a sample for testing. Please post numerical results, and keep us updated on the progress!