Whats wrong with my cat...?

Ok, My cat is no more than 10 years of age and I brought her to the vet yesterday, because I thought, for some reason she was losing weight, but the vet said her weight is just fine, and he felt her for lumps, and gave her a clean bill of health. But I forgot to ask him a question that I even wrote down before I went. The question is... why does my cat sneeze, only when I play with her, for example, letting her chase a string around the room, she goes BONKERS! But she sneezes every single time I play with her, but doesn't sneeze at all other than that, only when I play with her. I'm not sure, but it did seem to start after I gave her cat nip about 3 or 4 weeks ago, and I was thinking maybe she has some cat nip stuck up her nose, but is it possible after a month!? And like I said I'm not even sure if that is when it started. She's eating fine, sleeps fine and is obviously playful, I feel so bad. And I love playing with her, but I'm at the point where I almost don't want to because I don't want to see her sneezing, very strange, any possible causes would be greatly appreciated, her name is Pebbles, and she's a Money Cat. Oh and the vet a few years back did say that she has allergies if that makes any difference.