Well... it could be either but if you have built up trust with your dog and it seems clear that your dog understands this is a game then it is likely that your dog is enjoying it. If your dog likes the game it might even try to get you to engage in the game. Sometimes when dogs have the "zoomies" they run with their hindquarters down... maybe this is what you are seeing. If it is it is usually pretty clear the dog is having a ball (although some dogs do zoomies for stress relief too... but I think it feels good to them in that situation too... just the stress didn't).
If your dog is hesitant to come to you after, is "apologetic", or seems to be ignoring or avoiding you after it probably doesn't like the game. Sometimes I think it is easiest to see if a dog is stressed by observing it's facial expression.
Here is a post about playing with your dog by a well-known behaviorist Patricia McConnell. She blogs about dog behavior a lot so if you are wanting to learn more you should definitely read her stuff. She even has a DVD out specifically about play both between dogs and dog-human play... I haven't watched it but what I've read from her books I bet is is great.
From my personal experience... out of my four dogs, two of them like to play this game with me the other two would rather not. I know that the two like it because they give me "play bows" to get it started and have loose jawed happy smiles when they are playing the game. One of the two that doesn't like it would sit down or walk slowly away with her tail down and not try to engage... the other one just doesn't have interest in playing much anyway and wouldn't show any fear or interest at all when I do this.