How Long Should This New Piercing Hurt?
If you've just gotten a new piercing, you can expect it to hurt for a day or two - you just went through a medical procedure, after all. But after that, if it continues to hurt, or if it actually gets worse, that's a pretty good sign that you've gotten an infection and need to see a doctor for some medicine. It's a common occurrence with belly button rings and tongue rings, and is easily fixed. If you've had the piercing for awhile, you may be suffering an allergic reaction to the material. Try taking it out for a day or two and see if that helps.
After you have a new piercing, it will likely hurt a bit for the next couple days while your body gets used to having it. The sharp pain should go away fairly soon though, and the dull ache should go quickly after that. If it stays painful though, or if it actually gets worse, you may have an infection. Another sign of an infection would be swelling around the piercing, or a pus discharge. If you see any of this, go to the doctor right away and have it looked at.
If you've had a piercing for awhile, on the other hand, one possibility is an allergic reaction. Is the ring you have in new? Is it a new metal? Did you just change it? Try taking it out for a couple days, or switching to something safer like sterling silver, and see if that helps at all. If not, again, a doctor's visit may be in hand.
It's safe and fun to have a collection of cheap belly button rings or nose rings to play with, but if you feel constant pain, there's something wrong. Get to the bottom of it immediately, before you cause any serious problems.