If it’s simply good coding practice, then you’re fine.

If you’re cloning an API, that might be a violation. If you’re copying blocks of code, that’s definitely a violation. The fact that someone else actually wrote it in the first place is also an obvious signal that the code went from work to your personal project, and not vice versa.

If you’re still working there and want to continue to do so, I’d suggest reviewing your employment agreement and NDAs. Some employers have you sign an agreement that anything you write while employed belongs to them, others explicitly prevent you from using any ideas/use cases that you’re working on at work for outside applications.