At rest my oxygen sats are 98-100%, but after a recent stress test they dropped to 87%. what causes oxygen sats to drop after exertion?

Supply and demand: The exertion increases the demand for oxygen in many of your body's systems perhaps. Thus, unless the supply is also increased - there will be a (hopefully brief and moderate) shortage. With the procedures you've been through, be sure to ask providers for clear guideline for exercise and self-monitering. When to stop and when to call for help. Ask until you understand. I hope it goes well.