I have a 2.5 desexed make border collie. I have had him since 10 weeks old. Have done basic, intermediate, advanced training and agility. He gets love of love, attention (inside outside dog) and exercise both mental and physical. We walk 2klm to the beach 3 to 4 times a week where he is very owner/ ball focused but also very social friendly. He has always been a little anxious when dogs come up and bark at him in play or big dogs ie Great Danes but never really barked or growled I would just keep moving him thru the situation as I could see he was not comfortable. The last 2 weeks I've noticed he will sniff/great dogs like normal but if the dog then initiates play or chase he will growl and move towards it as if to move them on. He has never growled or barked at a little dog or puppy which he did today. Any ideas why this behaviour may be occurring? He can chase a ball etc at a million miles per hour and seems his normal self at home.