What Causes A Splitting Head Ache?

I woke this am with a fasting bs of 166 and had not eaten in 12 hours when normally I am inbetween 70-80. I had a splitting headache and was dizzy and shakey inside. I ate 1 piece of chese and 2hr later it was at 94. Also if I eat any carbs at all I get so tired that my eyes are rolling in my head. These last 2 weeks my sugar has been up higher and then as low as 65. I was told by my Doc I have prediabetes so I have been working on diet and eating according to what level my sug is at before. What is happening as I was doing well until a few weeks ago?