My sprite has weird gray polygon background, what's wrong???

Okay so I’ve made an animation for my sprite and I applied a material with a mobile diffuse lighting thing to it, and then in started having this weird gray polygon thing behind it in every frame (though the exact shape varies a little in each frame)
Here’s a picture of what it looks like:

I think it was because I was using the wrong shader. I was using the mobile/diffuse instead of sprite/diffuse