What Causes Sudden Rise In Blood Pressure In Elderly Person?

this morning i almost passed out , i was sitting and reading a book. also when i got up my legs where weak. i decided to get my blood pressure checked my reading was 174/81 and my heart rate was 71. i am not on any blood pressure medicine. my blood pressure has been on an average about 140/66 i am 78 yrs. old, about 149 lbs. i do not smoke and drink wine occasionally.