I'd think seat level could make a big difference, too. I wonder if the seat has scooted down a bit. Could that be?
Ha! Good guess, Rosie! Found this on Bicycling.com:
When You Say Ow! My...Knee
Try Tweaking Your...
Saddle Position: If the backs of your knees hurt, lower your saddle. Pain in the front or sides? Raise it. If they hurt on the outside, try bringing it forward a bit.
"Pedal Without Pain" (Halfway down the page)
It's certainly worth looking at!