Hope all turns out well, deafbat!
When my dog was just 3 mths old, she did the same thing. I thought she was able to handle the vigorous 'play' she was indulging in but apparently not. Came in the house with back right leg limping. I palpated every aspect of her tiny body and could find nothing wrong, and never got anything more than licks. Ugh! Bad mom. What guilt. Off to the vet immediatly as I was sure I had killed her but didn't know how! My vet is fabulous...gave her a complete thorough body massage, focusing on the injured leg and said it was a pulled ligament, probably groin which ran down into the affected leg, and it would heal with time and rest. We didn't get xrays as I trusted his diagnosis. He was right...time and rest cured the gimp. And funny, my 8 wk old BC, did the very same thing last wk (3rd day with me!) chasing the 1 yr old. I did the massage myself, which she loved, and made her 'chill' for awhile. I believe she is cured as well.
Sure does scare you tho. Prayers are with you and your pup for a speedy recovery!