My sister started eating paleo a few months ago and has noticed that her belly button piercing (of which she's had for 7 years+) has started to become really sore and irritating. To the point which she has to take the bar out sometimes (she has lots of different bars so its not one particular one).
She may have gotten something in her piercing to irritate it. I've had my nipple piercings for 8 years, and on occasion something will get under there to make it sore. Regular cleaning tends to nip that in the butt.