Glad you got a vet referral. The yelping does change the picture a lot! But it could still just be sore muscles from swimming -- that is an entirely different movement than walking and she might be a little sore just as we are the first time we swim any length after a long hiatus. My muscles got sore just reading about all the walks, swimming and stairs.
Still, you want to be sure it was nothing relating to the joints, especially since she had the little side adventure down the stairs. If nothing else though, you will feel better after a vet examines and hopefuly finds nothing serious. Then you can enjoy your visit and help Zoe relax.
I am surprised she could have gotten so much flea "dirt" (not sure what that is) and fleas in one day by just being exposed to them during one walk. I have never heard of anything like that.