Why does my chest hurt so much?

Ok, so last night I accidentally slept on my stomach and I usually don't do that. I woke up and my chest hurt when I took a deep breath, it's not a sharp pain, it feels like someone is sitting on my chest and throat. I don't have asthma (I don't think), and I'm scared it might be pneumonia. I'm 12, it actually hurts in my upper chest and into my throat. I don't feel like I have a fever either. But it is a VERY debilitating pain. But when I lay on my side I can occasionally take in a deep breath. What could it be??