Inuslin gone/going off

Does anyone know can the same cartridge/vial of insulin go off 'a bit' and work some of the time and then not be as effective for some of the time? The reason I ask is that I have been struggling for the past week and a half - sugars have been mostly high but on Saturday and most of Sunday they were great. I was using the same inuslin vial as I have since 14 August and when sugars were ok again on Saturday I used the same insulin for my cannula change. Low and behold when I've started a new vial this afternoon sugars have come down to between 4.9 to 6.5
It could be a co-incidence and the infection that the doc this I have is finally going but I did wonder about the insulin bit.

Thanks Noblehead - I think what I was tying to get at (and maybe a question too far!) was if it had degraded would it be bad all the time or could you have good and bad blood sugars intermittently with it!! As I say, probably a question for the scientists!! :roll: