It depends on the exact wording of your employment contract.
If it only covers work you did with company property, you're lucky.
As far as the law goes, in California (per Labor Code section 2870), employment agreements are allowed to assign the employer IP rights (including copyrights and patent rights) to inventions that satisfy any of the following:
- "[Use] the employer’s equipment, supplies, facilities, or trade secret information"
- "Result from any work performed by the employee for the employer"
- "Relate at the time of conception or reduction to practice of the invention to the employer’s business, or actual or demonstrably anticipated research or development of the employer"
For my part, the employment contract I signed is maximalist: it basically paraphrases the above language and says that anything satisfying any of those conditions is assigned to my employer.
The third condition is really the kicker. If you want to use your code at work, then the code probably "relates to" your employer's "business" or "actual or demonstrably anticipated research or development". If your employment contract is as maximalist as mine (and there's a good chance it is), then that's enough to make it not yours, regardless of who (if anyone) forked it.
In my case, my employer is large enough that it's hard to imagine software that doesn't somehow relate to its business…