Take a look at your employment agreement. It should have a clause in there detailing exactly how the IP is appropriated. Some companies' agreements say that they own ALL the IP while you're working for them, even the work done in after-work hours. Some say that only the work done on their computers belongs to them. Some agreements are ambiguous on purpose so they can come after you if you leave and they think you came up with stuff while working for them (which is very common... you see something your current customers need and you decide to go solo). Also, where do your customers come from? Are they the same as your employer's customers? They'll come after you. Are you competing with your employer? They'll come after you! Have you signed a non-compete? If you're in a similar/same area as your current employer, they'll come after you.

Anyway, this can become a huge mess... talk to your lawyer.