There are several types of vision disturbances such as halos, blurred vision, blind spots or scotomas. But I am not sure what happens to your vision since you didn't give me any information about it. And usually, the rapid vision change are possibly caused by some vision problems or eye diseases. Vision problems such as presbyopia, myopia or astigmatism can cause blurred vision. Besides, eye diseases such as glaucoma, cataracts and Optic neuritis may also cause some vision disturbances. So, you'd better take an eye exam so as to know what really causes the rapid change in your vision.
I guess it is one of the symptoms of iron deficiency. If I don't guess wrong, what you are suffering from is not only just rapid vision changes but also being sleepy usually, am I right? However, you don't need to worry if your prescription will change with your rapid vision changes. In most cases, this won't happen. So you can rest assured. Certainly you can go also go to have your eyes checked if you really worry a lot about this.