Help urgently needed with Nikon D7100 camera?

I'm in urgent need of anyone who can help me fix my camera in any way. I got the Nikon D7100 model last November (2014) for my birthday and it's served me brilliantly well. However, in the past month or so, I've noticed that the photos have become increasingly grainy, especially in sport mode and in low light. I have tried everything; cleaning the lens, changing the lens, cleaning the view finder, cleaning the monitor, cleaning the part where the lens clicks in, changed the exposure settings, everything I could think of but so far nothing has worked. I really want to progress with my photography and I have an Instagram account with over 5k followers, but if I have a non-working camera, I don't know what I'll do! Any help, big or small, is very much appreciated!

Is it grainy because of high iso? If it is try to set the iso not too high, even your camera could handle high iso good enough.