Hello mruffruff. There could be many reasons for weight loss. Unfortunately, it is a very common side affect of many diseases. Have you noticed if she's drinking more water than usual? Diabetes could be a cause of weight loss, and extra water intake is a sure fire sign of that. Also, could be be having a reaction to fleas? It could be that she's allergic to them, and they're causing her to feel ill and not want to eat.

A blood workup is probably the best way to go to determine what is really going on. I hope she improves and gets better soon! Good luck and keep us posted!
**hugs**