According to the American Institute of Cancer Research there are a few animal studies show that flaxseed does not interfere with tamoxifen’s actions and may have enhanced effectiveness. However, there are not results of clinical trials on humans for the use of flaxseed during tamoxifen. Therefore, decisions about flaxseed use should be discussed carefully with a your physician.
If you fear that the flaxseed will potentially increase your estrogen levels, don't worry, there is no evidence that it will increase your risk of reoccurence of breast cancer. So it should be okay for people who wish to consume flaxseed as a source of omega-3 fat or dietary fiber.
The following link is to a 2010 article about flaxseed, tamoxifin, and breast cancer, and it does not to appear to be a problem either. It actually might even help:
The following link is an in depth article from the American Institute of Cancer Research discussing the use of flaxseed and breast cancer.