I have no idea what could be wrong with her, but I would say that a daily, if not twice daily, change of the paper in the cage is a must have in any scenario, let alone the one you have described.
The reason the answer is always 'take her to the vet' is because people can only give accounts of a situation that they have encountered which *might* have the same symptoms but doesn't mean that it is the same illness and much more likely, the vet is going to need to administer something that you can't treat at home.
I hope as you recognise that she needs a vet, that you have gone ahead and booked an appointment as you know our stance is on promoting best practices which includes taking sick budgies to the avian vet when required.
Do you plan to take her to a vet? It was unclear in your post. You acknowledged that was the correct thing to do, but not if you were going to do it.
Besides from that, everything else you can try to boost her is also already on the forum, if you look through the forums.
Again, it seem important to emphasise that she should be going to an avian vet for a correct diagnosis, rather than any guesses any staff or members here can be making.