Incorporating Chris's comment as an answer:
You'll probably need to seek professional advice. This situation can be complex. What citizenship(s)? Do you provide contract services as a corporation, or as a sole proprietor? Do you have other clients? Any chance you'd need to physically be in the U.S. to do any part of the work? Is your client withholding any taxes and/or providing you any tax slip? Were you required to provide the client with any tax documentation?
Here's a thread I found at LinkedIn describing how complex such a situation can be.