Insufficient data!!! What is your age, sex, medical history? Was your BP 120/100 once? All the time? Were you standing, sitting, lying, ...and many other factors. However, generally speaking, it is the lower number, the 100, or the diastolic part of the BP that may be a problem. The top part or number of your BP indicates the pressure during systole
, or "when your heart is pumping", the 2nd number indicates a more constant pressure your vascular system is under, and 100 is indeed high. Who took this BP? With the right size cuff? Listen, your BP changes all day long , don't freak over what may be an isolated incident. Then again, it would be wise to monitor this situation via your MD, who can of course then take appropriate steps to help.