Young dog died suddenly?

Hey, my 10 month old Australian shepherd girl died suddenly. She was running after some dogs, we called her back and when she came back she let out a mournful howl, keeled over, had a l discharge with puss in it, and then died in just a few more seconds. I don't know how she could have died, she seemed fine no symptoms of anything. Seemed very healthy and now she just died suddenly. How could this have happened? I appreciate any help, my whole family does :)