Canon Rebel xsi prints look blurry?

Although I'm not a professional AT ALL, I do have a Canon Rebel xsi and am TRYING to learn a bit more about my equipment. When I first got the camera, everything was wonderful. Now, after having it for two years, I feel like something has happened, perhaps I hit some button unknowingly. Anyway, all of my pictures look great on my computer, not blurry AT ALL. However, when I print out anything over a 4x6, the pictures look blurry/degraded in some way. I don't understand. I shoot in the simplest auto settings and I've checked the image quality. I have it set to the highest it can shoot (the little arc with the 'L') with the exception of shooting in RAW. So, why do my images look great on my computer but crappy when printing out? Also, I don't know if anyone knows the answer to this, but I used to be able to shoot continuous in sports mode without getting ANY blurry shots. I felt like a professional; every single shot looked like it could be in SI! However, now, nearly EVERY shot I take (in sports mode) looks blurry, which seems a bit ironic to me.