Assuming you are otherwise healthy, your BP is not that low for a healthy, young female. One blood pressure reading does not indicate much of anything, especially on a home unit.
Drinking water is always good advice.
If you are concerned, most physicians and clinics offer free blood pressure checks to their patients. Trends are important with BP's, ask your provider what he or she recommends for you regarding frequency of measurement.
If you have recently started, stopped, or changed a medication, call the prescriber and ask if that may have contributed.
You may be a little dehydrated. Just drink some water.