My Nikon d5100 is producing a lot of noise in my pictures. How can I fix it?

This just happened in the past week and I used to be able to blow up a picture to a 16x20 with no problem, but now it's hard to even get an 8x10 to look good. I don't understand what happened. Can someone shed some light on this problem for me? It also seems to be happening with more than one lense. Thanks!

Added (1). Edited to say - thank you for the ISO suggestion… What about the high ISO NR? I believe when I was shadowing a fellow photographer he had me change it to low from normal. Could this be the problem? I will try this tomorrow I think.

Check your ISO. If it is set too high, you may be getting excessive noise.

My guess is that you have the ISO set to automatic. In low light conditions, the camera will automatically raise the ISO to prevent blur from slow shutter speeds when hand held.

Take control of your camera and take both the white balance and ISO out of automatic and start learning how to match the white balance to the lighting and your ISO to provide shutter speeds of at least 1/250th second until you learn better how to hold the camera steady at slower shutter speeds

Other than the iso I would recommend a faster shutter speed. I don't know about a nikon d5100 but usually you can change this on a little scroll thing on your camera. When you want a slow shutter speed put it on a tripod to get rid of movement and blur. Also just set it on manual this will usually be okay for casual pictures.

For many here ISO is the Elephant in the room!

The higher you set the ISO, the more sensitive your camera will be in low light but also the more random noise you'll get in your images. To be honest the result of using "High ISO Noise Reduction" is usually worse than the noise you are trying to remove!

We can talk about this till the cows come home, but the best thing to do is to have a little play with your camera!

Set up a little scene indoors, turn the flash off, and set the camera to record in RAW and the mode to Aperture (so that it auto sets the shutter speed). With the camera secured on a tripod take multiple shots of the scene you created, but each time increase the ISO - take note that the shutter speed will decrease as the ISO increases. When you've done, examine the photos you took using the software that came with your camra looking at the ISO, shutter speed, and image quality.

Just a practical idea so you can get to see the effects for yourself.