There are many conditions that can result in blurry vision, some are life and sight threatening, others can be resolved with spectacles and or eye drops. One of the most common and serious causes is Diabetes.
Blurry vision is often one of the first warning signs of diabetes. Your vision may be blurry because fluid is leaking into the lens of your eye. This makes the lens swell and change shape. Those changes make it hard for your eyes to focus, so things start to look fuzzy.
You may also get blurred vision when you start insulin treatment. This is due to shifting fluids, but it generally resolves after a few weeks. For many people, as blood sugar levels stabilize, so does their vision.
If your vision is fluctuating, get your eyes examined.