Why is my healthy cat losing weight?

My 11 yr old cat has lost weight over the last year since she turned 10 in spring '10. There have been no other changes other than weight loss. She is energetic (if not more so with the weight loss!), eats well and bodily functions are normal. I did change her food last year from Iams Healthy Weight/Hairball Control to the Purina One brand with the same attributes. Should I be concerned? Her vet was wondering about a 1 lb. weight loss last year when she had a summer checkup, but I would venture to say now she's likely lost another 2 lbs - a lot for a cat who at heaviest was 11 1/2 lbs. I hate to take her to the vet now as she's not due yet for a checkup until late July, and she seems fine. Honestly, I can not say there is anything else of concern other than she seems to be 'shrinking'! Has anyone been through this? Any ideas? I'd appreciate any input - I don't know anyone who's ever had this happen.