I have noticed a pattern with my eldest dog (7.5yrs).  He has slowed down over the years and trots along when we are out and about with the other two dogs.  If he chases a ball or toy and moves quicker than normal he will, after 5/10 mins or so be sick out on a walk.  It is usually very digested food or bile as I never exercise after a meal (at least 3 hours minimum) if we leave the toys at home and he trots and walks about he is not sick.  He is on meds for his AF but apart from that is a fit dog. He did his last 25 mile walk back in Sept (now retired!) and was fine but he paced himself and did no galloping around and was fine.
Does anyone have any ideas on why this could happen after strenuous exercise?. 

it happens to people if they get over excited and strenuous exercise, i would imagine it would be same for dogs. have seen even younger dogs do it especially just after mating

if you met him, you would see that he really isn't an excitable dog, years ago more so but nowadays a chilled gentleman when out and about. Chases a ball when thrown even tho he knows my gsd bitch (greyhound in discuise) normally gets it first!

maybe he need a bit of food before exercise but an hour half or so before , one of my girl is older now and need something in her tummy or she brings up bile.