What Causes Sudden Onset Of Fatigue And Lightheadedness?

Hello, I am a 50 year old woman who has been told for many years by doctors and health care professionals that I have normal to low blood pressure. My blood work on my annual exams always comes back with a good bill of health and told to keep doing what I have been doing. Also, I am of average weight and have maintained my weight for years with a good diet and exercise. As of late I have been experiencing off and on a sudden feeling of fatigue and light headedness . This causes me to take a break from what I am doing, if I can, and sit down to rest. Either way, the feeling usually passes in about 20 minutes and then my energy returns and I can continue about my day. This has happened to me through out the past 5 years or so, although it seems to be happening more often. Some times 2-3 times a day with some days I am symptom free. This happens even after I have eaten a large meal so my blood sugar should be fine. This happens if I am in cold weather or hot weather. I have two brothers who have controlled type 2 diabetes as well as my deceased grandfather. Should I be concerned? Thank you, Rosemarie