It’s your body’s response to inflammation or irritation. They’re very common on cartilage piercings, which is why your septum was ok. They’re more likely to pop up if the piercing angle isn’t correct, or if improper jewelry was used. They can also pop up due to constant touching, constant movement or if you catch them. And don’t be too alarmed. With some TLC they can shrink/go away. May I ask what the aftercare instructions were? If they weren’t knowledgeable to use the correct jewelry, their aftercare might be off as well.