A woman's statement that she is niddah – even if she really isn't – is enough to render her niddah and require the complete taharah process. However, if she offers a valid excuse why she made that statement, then she may retract and is not required to complete the clean days and immerse. The examples you gave, as well as others (such as the statement was made in anger/they were in a fight, etc.), are brought down in the poskim. Some poskim do question whether a woman may retract if she declared herself niddah as a practical joke (and it wasn't obvious at the time that she was joking). The point is that a woman's word is taken seriously, and unless she offers a valid excuse, her word can actually render her niddah.
This matter should not be taken lightly, and even if the excuses mentioned are considered valid (i.e., she's sick or embarrassed or angry) a woman really should not rely on this tactic to push off her husband. Communication is so important in a relationship and it is much better for husband and wife to be straightforward with each other. Obviously we are not referring to the situation where a woman thought she was actually niddah and later found out it was an error. In that case there is nothing wrong with retracting, since she clearly had no intention to "trick" her husband.
Please feel free to get back to us with any further questions.
B'hatzlacha!