All right so first off I'm 13, so I know that obviously I'm not done growing yet but I have started going through puberty, if that changes anything.
Here's a description of my body:
I have narrow shoulders, small boobs, a large midsection (so a beer belly, muffin top, whatever you want to call it), a curvy butt, slim hips, my legs are short and slim, I'm kind of short/petite I guess because I'm 5'3''. My arms aren't particularly fat or skinny, they're average I guess and kind of toned but that's probably because of working out. And I don't know if this matters but I'm at a healthy weight for my age and height.
I thought I was a diamond shape, but then I read that diamond shapes have full breasts, which I don't have. So does anyone have any idea what body shape I'd be?

Pear?

What does it matter? It is going to change in a year or less. It could be diamond, pear, apple, brick s**t house, hourglass, or sausage depending on how everything evens out. The best reference is to look at your mom and your father's sisters or mother. That will give you a decent idea.