My cartilage piercing that was healed has randomly gotten infected again, why?

I have 2 cartilage piercings, the very first one I got was in 7th grade and yes they do hurt at some points, they do randomly get infected. It's been 4 years now and It's fully healed. My other one I did about 3 years ago and it randomly got infected I guess? like a week ago, It was like yours but the bubble wasn't as big, it was blood and all I did was squeeze the blood out in your case squeeze the pus out, squeeze it all out until the bubble is gone, clean it with peroxide or alcohol then use the sea salt solution. It should feel a lot better right after that, and by the next day it shouldn't hurt anymore.