First, read this: If I'm working at a company, do they have intellectual property rights to the stuff I do in my spare time?
Now with that information in hand, realize that depending on the property rights contracts you sign with your clients, you may or may not be able to do what you elicit that you're doing.
It depends entirely on the individual contracts you sign, also it depends on the similarity and if each piece is directly copied or written from memory each time. Especially the similarity part. Absolutely identical snippets from one to another when the first client owns property rights, would definitely be seen as a violation of an NDA in court at the least (copyright infringement/theft at the worst) most likely wherein you aren't allowed to disclose proprietary methods from your client to other companies. While you may have written their code, disclosure of said code in identical form from memory would be tantamount to disclosure of a company's strategic advantage from memory which you gained in a privileged capacity.
Again, this all depends on the contracts you sign, but pretty standard contracts would make a complete copy-paste illegal. However state laws may override certain forms of these things so if you are relying on yourself as a business owner to employ you, I would have said business owner consult a lawyer for advisement lest the business that employs you get into hot water.
Make no mistake however, these things are extremely common occurrences, and companies press charges on these things probably as commonly as the legal departments understand what a software developer's job even is :)