The main thing to consider is whether you signed any type of agreement that would
assign any copyrights to your employer.
If you are a normal employee, in general your work is considered the intellectual property of your employer (and employees sign an agreement to this effect). If you are an independent business (e.g. 1099), an agreement is needed to transfer copyright and/or patent rights from you to the company that is paying for the work. If they have a standard contract, see what it indicates about this. It may be convenient to tie transfer of the intellectual property rights to payment.
Absent any agreement to the contrary, the copyrights should be yours.
Lawyer
5 years experience in intellectual property law