Normally, unless otherwise agreed upon and specifically paid for, any artwork created by an artist working for a screen printing company belongs to that company entity. When you contract a screen printer to create art the art copyrights belong to the screen printing company since they actually originated the design. If you want to "own" the art work you usually pay a higher fee and negotiate that before you contract the work.
When a customer gives artwork and the screen printing company only does a separation of some kind and prints it pretty much as is then the person who originally created the art owns the copyright. The screen printing company owns the film, screens and digital separations unless otherwise stated by the screen printing company. But they do not own the copyright to the actual artwork.
The bottom line is if you need copyrights, control and ownership of a piece of art that you are contracting someone else to create under your guidance then you need to negotiate that and agree on it in writing of some kind. You may even want to immediately file a copyright with the Library of Congress.
However, this is not to say they won't be willing to work with you since you used to work for them. Given it was a pleasant experience of course
But I'll assume they'd rather print the job themselves.