Did my dog have a heat stroke?

Yesterday, my husband and I were outside doing work around our farm. It was in the high 80s but a breezy day, so not unbearably hot. We let our dog, Rocky, who is a boxer/great pyrenese mix out with us. He played, rested, drank water when we did, and seemed fine. Until later that afternoon, my husband hollered for him to come to us, and he wouldn't, it was like he was tangled in something. So I ran to him,and he was picking up one paw and putting it down but not going anywhere. My husband picked him up and put him in my lap and we drove to his water bucket. He staggered when I first put him down, so I helped him stand at his water bucket and he drank a good bit of water. He was fine afterwards. Besides his staggering, his tongue was rolled out and he had that helpless look in his eyes. Needless to say my eyes were filled with tears. This is his second spell with this. The first time was earlier in July; however, it was at night around 7:30 and the temp was 78 or so. He only ran around for about 20 minutes as my Mom and I rode the trail wagon around the farm, then it was the same symptoms. After water, he was fine then too. I called the vet the first time the next day and she said if he was fine, then there was no need to bring him in. But I am calling tomorrow and making him an appt. I just want to know have you ever had this happened and what was it. I also applied a cold rag to his belly for him and put him in the shade after he could walk, which was right after he drank his water.