Could be a bruise/broken blood vessels under the skin from all that running and playing or could be an insect bite.
Like Chi said just keep an eye and if it changes....bigger, swelling, puss filled, etc...or her behavior changes, make an appointment.
I'm sure it's nothing to worry about, just probably got a bite or bruise.
Take care :)
I could not answer you on this forsure, who knows?? Keep a good eye on it, if it intenses on size or irritation to the dog, definitely seek a vet.
Try to put some cornstarch the kind you cook with on it, it won't hurt the dog if he licks and see if that helps any...
Welcome to forum, and good luck, keep us posted and let us know whats up...