I saw the same thing just doing low carb. After some weeks of steady readings I was suddenly getting erratic results - but after a couple of weeks more numbers began to go down much further than I'd seen before and dropped down to under 8 mmol/l all the time.
My thoughts are that it is a natural response to being released from the error of high carb, and as it was the start of weightloss - I'm pretty sure it must have started about then, it indicates a return to normality rather than anything sinister.