There are many causes for your eyes changing. As we age, our eyes can change (one way or the other). Medical conditions, such as high blood pressure and diabetes, can cause such drastic changes as well. Eye conditions, such as glaucoma and (most likely) cataracts, can certainly change the prescription.
You should have a complete physical to check for any untowards medical conditions. Most likely the cause is a cataract. The first signs of a developing cataract is that it changes the focusing of your eyes. This will be followed in the next few years with a change in your vision (your vision will slowly get worse regardless of glasses or not).
Hope this helps and thank you.