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Your symptoms (nausea and stomach cramps) could be suggestive of pregnancy. Look out for other symptoms suggestive of pregnancy. Sometimes your normal fertility can return back soon after coming off birth control. Urine pregnancy tests can show false negative results, hence cannot be relied upon for accuracy. Blood HCG levels are accurate and confirmatory. Go ahead with a visit to gynecologist for examination and tests to confirm/rule out pregnancy.
Take care.