Well, even if you didn't have your bun out in that pen that lady shouldn't have had her dog off a leash anyways! What if a wild bunny or a squirrel ran out in front of her and the dog ran after it?
The dog could have gotten lost or hurt or hit by a car! Some people just don't think.

And by the way, you are definantly not a bad bunny owner. At least your bun was in a pen and you were watching him to protect him from danger.