Your dog will be fine! The kind of mushrooms that are on pizza are no problem for your dog, but onions should be avoided. You do need to avoid mushrooms outside though. You can see a full list of outdoor plants to avoid at www.runningthepack.com/blog
I agree with Lisa Marie1, My dog is lactose intolerant, and once I gave him a tylenol[Do not do this] because he was in pain and I couldn't get him into the vet for awhile, and it did not kill him. He is a small dog too. He has grabbed an occasional cheese nip, when we couldn't catch him, and he is O.K.
I have heard mushrooms are bad for dogs and can be poisonous, yet if it were just one, I think she should be ok. I would keep an eye on her, and make sure she acts the same, and continues to eat and drink. Call your local vet, and ask them, they will be able to tell you the best advice. Good luck.