At the time at which you have recorded this BP value, did you have any giddiness or weakness etc? If not, then most likely this is a false value and you need to recheck with an appropriately sized cuff. If still persistently low, then you need to check BP in both arms and both legs. If there a discrepancy then you need to be seen by a cardiologist physically to rule out diseases like Takayasu’s arterirtis, Coarctatuon of aorta, cervical rib, peripheral arterial disease etc.