Unknown Device on My Account

[RHawthorne 0]

Every few weeks or so, I am not allowed to access my account until I unsync a device. I have only used my account on my iPhone and my laptop computer, and nobody else is accessing my account that I know of. I had shared the account with one other person over two years ago, but I know it's not her that's accessing the account now. The third device that keeps showing up is a Motorola phone, and I don't own one, and the person to whom I had granted access to the account has never had one; she has an iPhone, and has for years. The only thing I seem to be able to do is unsync the third device, and I don't see how I can permanently deny access to that device or the person using it. I have no idea who that person might be, or how they keep accessing my account, or what kind of information they might be seeing, or why they would want to access my account. This is highly concerning and irritating for me, and I don't know what I can do. Any responses would be greatly appreciated, as I can't seem to find any topics on this site that seem to address my concern.