Shutter Priority -setting shutter speed

I have a Nikon D7000, for about 6 weeks.  I have been experimenting with using the shutter priority mode for taking pictures at sporting events.  I was able to set the shutter speed faster than 1/250, but now am unable to set the shutter speed to anything faster than 1/250.  I did take photos at faster shutter speeds in shutter priority mode a few times before and noticed that occasionally I could not adjust the shutter speed but then the problem would "resolve itself".  tonight, I am unable to set the shutter speed faster than 1/250 and it is not resolving.  I can only think of two things - 1) As a novice, I have entered into an unknown mode on the camera that prevents setting the shutter speed,  2) the camera is defective, or 3) I just don't know what I am talking about (reference my screen name ).  does anyone have any ideas  on how to troubleshoot this?