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 also required for a warrantless arrest. Probable cause is an objective standard rather than a function of subjective opinion or suspicion not grounded in fact or circumstance. However, the facts or circumstances need not be of the nature of certainty necessary to establish proof in court.