Why is my Nikon D3200 Taking grainy pictures?

I recently purchased a Nikon D3200. I'm shooting a short film and so far this has been the best camera I've used for film making (A major step up from a camcorder.) At first my D3200 kept taking great pictures, but slowly it started getting worst. On the second day my ISO setting didn't allow me to put it on auto ISO, and now about 1 day ago it started taking grainy pictures. What's going on? I checked the setting and even reset them. As for picture I took for my actors durring my short film, they we're grainy especially when it was really sunny and not one bit sunny outside. I need it to shoot some good pictures in broad daylight. What's going on?

(My ISO was at 100 when I shot pictures.)

First guess?
User error.

If you post an example it might help diagnose your problem.

Sensor could be overheating.

Also, usually the aperture will be set at one setting once you start filming. Go to A mode and set the aperture to the lowest setting before you go into live view. Regardless of the ISO settings, the camera will raise it during filming if there isn't enough light.

What you are calling "grainy" photos is actually noise.

Have you been using the cameras lightmeter to pick your exposure?

Here is a link that will help you balance the ISO, shutter speed and lens aperture.

If you are using the automatic or programmed exposure, stop.

You have a fully adjustable camera so it is now time to start using it as such.

If you are using the camera to shoot video, it is clear that you don't know that the only control over the exposure of the video is by opening up the lens aperture and raising the ISO settings.

When in the video mode, the ISO automatically raises once the maximum aperture opening is reached. Your problem seems to be that you do not have enough light on you set (or scene) so you camera tries to get a good exposure by automatically raising the ISO.

What you need to do, is what film and video production companies do. Add light. The most simple way would be to replace 40 and 60 watt bulbs in you scene with 250 watt bulbs. This way the ambient light scheme is not disturbed but your camera will be able to produce videos will less noise

Grainy pictures is a sign of high ISO. The camera is thinking there's low light. Check your settings again. Some modes do not allow manual ISO setting. Check the manual to be sure.