People can have different reactions to a piercing, which is why you need to keep an eye on it. As lilibees says, you could be allergic to the metal.
I found with my nose piercing in particular that it would get red and sore if I touched or played with it too much. If you're really worried:
- put a little salt into some warm water,
- take a cotton bud
- clean around the piercing slowly and thoroughly.
Do this twice a day for a few days and see if it helps. This helps clear the piercing out and prevent infection from setting in :)