I sometimes get it. But most of the time I am OK. Does anyone else have this problem?

My internist told me to get a blood pressure cuff to learn what my reading is when I feel lightheaded and weak. She does not believe that I need to monitor the blood pressure all the time. My top reading at dr's offices and during blood donations are usually between 105 and 115, but I have been known to have 100 and even 90.