I am a thirteen year old girl, and at random times my heart/chest starts to hurt when I breathe. It just comes and goes when ever it wants to.
I've told my mom, and she says it's that I have too much air in my lungs. But my left side ONLY hurts, not both. It's been going on for almost about a month. And I don't know what it is.
WHAT'S GOING ON? Please help, the pain just gets worse and worse.

No offense to your mom, but she has no idea what she's talking about. Tell her you really do need to see a doctor. If she won't, find someone who will take you to a clinic. If school starts soon for you, see the nurse.
No one online can possibly know what it is, and any guessing at this point would be fruitless, from simple heartburn (gas in the high part of the stomach right under the heart) to pneumonia all the way to something rare.