Open Question: Am I underweight? Or is it stress?

I'm a boy, 13, almost 14, weigh 100 pounds, and i'm 5'l 1/2. Everytime I get up and pretty much do anything, my heart starts to beat faster and harder. I also do it when I think of anything that upsets me, like anxiety or something. My mom says i'm underweight, and that's why my heart's beating so fast. Anyone have any ideas?