That depends on the agreement you signed. It's common for the invention assignment agreement you sign when you start working at a software company to cover anything you do, even on your own time with your own equipment, if it "relates to the Company's business, or actual or demonstrably anticipated research or development of the Company", or similar language. If your employer is a large company with its fingers in many pies, this could apply even if the work has nothing to do with what you yourself do for them.

California is unusual in that it has a statutory protection against the most expansive assignment provisions, but even it allows that provision. http://www.leginfo.ca.gov/cgi-bin/displaycode?section=lab...