emilycass Do you still need help? If so, please test a new save without the Pets expansion present. Wherever the game is installed (Applications > EA Games by default), you'll see a Sims 3.app (with the green plumbob icon) and a "The Sims 3 Packs" folder. Open the Packs folder and move the EP05 folder to the desktop, and Pets won't load.
You do need to test a new save, since a save that has previously used a certain expansion won't be viable without that expansion present.
I'm not saying you can't play with Pets installed, only that this is a useful test. If it helps, there are ways to try to address the issue while still allowing you to play with the EP present.