I need some advices!

My 6-7 months puppy was growled at and put on its back by a pitbull. Luckily nothing else happened but he got really scared. Anyway I believe that triggered something in him because now he lounges at other dogs and growls at them (not all of them but it happened twice in 2 days). Yesterday I was playing with the Frisbee in the park and a younger retriever , perhaps 5 months came towards my dog and he got very defensive with his toy ( he never showed resource guarding before ) and today once again he got playful with a puppy which was smaller than him, they were chasing each other and suddenly he lounged and growled at the puppy, almost starting a fight. My dog used to be very timid and afraid of everything but now he gained a lot of confidence.

My 6-7 months puppy was growled at and put on its back by a pitbull. Luckily nothing else happened but he got really scared. Anyway I believe that triggered something in him because now he lounges at other dogs and growls at them (not all of them but it happened twice in 2 days). Yesterday I was playing with the Frisbee in the park and a younger retriever , perhaps 5 months came towards my dog and he got very defensive with his toy ( he never showed resource guarding before ) and today once again he got playful with a puppy which was smaller than him, they were chasing each other and suddenly he lounged and growled at the puppy, almost starting a fight. My dog used to be very timid and afraid of everything but now he gained a lot of confidence.