I'm not a lawyer, but unless your former employer has granted you permission to share this code with those you are showing it to you are probably in violation of your original contract. If you wrote code for a client and you think that you have a legitimate copyright to that code you should see a lawyer about it. Most of the time code written as a contractor is "works for hire" and you have no such rights. The code is not copyright because you wrote it under a contract for some other legal entity.
I would stop showing any code to potential clients, though you should be able to talk about most of your previous experience in enough detail to demonstrate that you have what they want.
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I'm not a lawyer, but unless your former employer has granted you permission to share this code with those you are showing it to you are probably in violation of your original contract. If you wrote code for a client and you think that you have a legitimate copyright to that code you should see a lawyer about it. Most of the time code written as a contractor is "works for hire" and you have no such rights. The code is not copyright because you wrote it under a contract for some other legal entity.
I would stop showing any code to potential clients, though you should be able to talk about most of your previous experience in enough detail to demonstrate that you have what they want.