Is my dog hurt? I can't tell!?

Today my friends and I were in my backyard on my swing set and my mom had let our dog out. While my friend was swinging very high and pretty fast my dog ran behind her and when she accidentally hit him, he tumbled and rolled over twice. I can't tell how bad he got hurt, but he is limping a little more than usual and walks up the stairs at a very sluggish pace. I was wondering if anyone knows what could be wrong. He also has been panting a little.