It doesn't take much for glasses to give you problems. One of my (round) lenses once got loose and turned a few degrees, and that was enough to give me headache after headache. The correction was correct, just the cylinder of one lenses was a bit out of whack.

Also, your vision might be changing, Or you might be at that age when presbyopia starts setting in and your eyes can still see fine up close, but they're straining at the limit.

My experience with vision changes has always been that it was fine for years - or decades - and then all of the sudden, over the course of a few months, I'd start getting problems. You might be at that point too.