The foot is always changing just like the rest of our body. So it's not unusual for our foot size to change. Just like our shirts' neck size changes as does our waists' belt size, so might our shoes' size change. That's why it is important to get your feet re-measured.
A mis-fitting shoe can cause a lot of problems. Make sure that your bunion matches up to the widest flare of the shoe and that strumming across the front of the shoe allows a rippling of the material. Lastly, make sure that there's enough room in from of the toes so that they may wiggle freely.