Is it really that unlikely that my eyes have changed in 3 months (details inside)?

I’m 16. I got glasses for the first time about 3 months ago. It was great, because it explained away some health scares I was having at the time. I wear the glasses all day everyday, like I’m supposed to, but in the last few weeks, I’ve noticed that I feel like my eyes get tired way to easily and can’t always focus quite right. I thought that maybe the prescription wasn’t exactly right and I just needed another exam, but my parents and the eye doctor all seem to think that’s not very likely.

So is it really that unlikely? Isn’t it possible that the first lenses worked at first, but then turned out to not be what I actually needed? Something like that?