There are many reasons you might miss a period. Simple things such as stress or exercising too much can cause a missed period and many girls are irregular during the first few years of menstruating. But if you've missed a few periods, be sure to see your healthcare provider.
Courts usually find probable cause when there is a reasonable basis for believing that a crime may have been committed (for an arrest) or when evidence of the crime is present in the place to be searched (for a search).
Probable cause is not equal to absolute certainty. That is, a police officer does not have to be absolutely certain that criminal activity is taking place to perform a search or make an arrest. Probable cause can exist even when there is some doubt as to the person's guilt.