It is especially obvious when you are getting a new prescription or a new frame much larger or smaller than your old ones. Even for the new glasses with the same prescription in a similar size, your eyes have to adjust to the new situation of the lens to see it clearly and perfectly.
It's not unusual to have a headache the first several days you wear your new prescription glasses, but it subsides after a one-week adjustment period. The progressive or bifocal lens would have a longer adjustment period that is mostly two or three weeks.