Shell, when my Molly got older she also lost lots of weight; esp round the back end. Admittedly she didn't eat vast amounts but in her prime she was quite a chunky cat but at the end she was only 2.1kg so really nothing but skin and bones
How much weight loss are we talking about by the way?
Also, as a matter of interest when was the last time the vet ran some senior cat bloods on her to check that everything is working as it should? If it hasn't been too long ago (ie within the last year) and all is fine then I wouldn't worry about it; it's normal for older cats to lose weight and if she's happy and healthy otherwise then just enjoy her.
What food is she on?