Belly Button Piercing

Hey I’ve had my belly button pierced for almost 2 years. Healing was great, first year was great. Every few months now this pops up, it may get a little bigger each time not sure though. It gets cleaned with saline and in the shower. It doesn’t usually hurt and goes away in like 4 days or so.

I also don’t think it’s rejecting because this has happened many times now, I don’t take the piercing out and it’s always gone and fine again in 4 days or so.

Just wondering why this keeps happening and if there’s something I can do to prevent it.