Why is my FBS fluctuating. with exercise, strict diet control for two days it becomes 80. No exercise 135 etc?

Two and half months before I made a routine check up and my fast blood sugar level became 176. My BP was 160/110. I reduced weight drastically i.e 11 KGs and it is now consistently 120/80. I took BP tablets for 8 days and stopped it by Doc advice. A re-check of FBS the next day, it became 165. I had another one on the 5th day and it became144. The Doc said, "you can control this by diet". I started taking diabetic diet and it is now consistently below 95. And yet I am disturbed for I am not sure whether or not I am a diabetic. Am I diabetic? If I am a diabetic, how could it go so low within a week time? It had even gone as low as 59 and 60 in two of the tests. It was 95 only once and I had eaten pizza the previous night. Is it a pre-diabetes or a diabetes case? Is it wise to start taking medicine at present? should I consult more than one Docs? I am at a loss. It could be of some interest for you to know my brother died of diabetes melitius at the age of 34. I am 54 yr old.