Based on your description, there’s no question that you were an independent contractor. Of course, this assumes that no paperwork exists, including a contract or NDA, that legally binds you to your former client.
Regardless of what this client intends to do with your work, because you do not possess a written contract stipulating the exact nature of your working arrangement, this oversight could pose a major problem for you if you allowed this to go to court. The length of your working relationship may also prove problematic.
Rarely are such threats anything more than hot air, but you still need to be careful. Most people who threaten legal action do so because they do not have a legal leg to stand on. However, if this former client has the desire and capitol to fight you in court, you could be ruined financially even before this case ever found its way into a courtroom. Keep in mind that laws governing work-for-hire vary from state to state. Some states, like Georgia, do far more to protect the rights of the small business owner.
If the work you did for this client is important enough to you to protect:
1) You can attempt to resolve this matter on your own. In writing. Certified with Return Receipt (or, FedEx).
2) Contact a lawyer who specializes in Copyright Law.
Only you can judge whether or not pursuing legal protection for your work is worth the time and cost. Just be professional and smart. Laws governing creative rights are written obtusely for a reason and you will need someone on your side who is experienced if you choose to handle this matter legally.
3) Chalk it up to experience and let it go. Just make sure you use a contract for long-term engagements in the future.
and, add this client to the Stop Work-For-Hire List.
