Bring it to a tire shop and get it checked. You could have a smal nail in it, or the valve stem could be faulty. Hopefully it is something simple like that.
Highly recommend discount tire. They're actually the ones that installed my custom wheels and tiresTake it to a local Discount Tire or America's Tire. They will check it out for you for FREE.
Go to www.tires.com for a store locator.