Breast Implant moving 3 years postop - what could this be?

Based on what you describe in your question, further investigation is warranted. My recommendation would be to obtain an ultrasound first to enable a look at the implant, the muscle, and the capsule. Once this is complete, and based on the results, we can determine the next steps.

Your implant may be pressing on a small sensory nerve and causing some discomfort. At one point, you may have torn the capsule some and exposed this little nerve more to the implant, or just tore the capsule somewhat and it will heal. It should slowly improve with time on its own.