She has been losing weight, she has loss of apetite, she sort of jumps when she walks etc. I can't take her to the vet, but are there any things that I can do at home to make her better? She's still not in a very, very bad condition, she still squeeks and she doesn't look so bad. What can I do?

vitamin C. Go to a vet and purchase vitamin C snacks they sell. Or go buy chewable vitamin C, crush it up and sprinkle some on a small piece of fruit if you give him too much, do not worry, the body will simply excrete the extra. Piggies need 25 mg of vitamin C, when they have scurvy, give them extra.

She needs a good diet, vet visit and overall good care. Please read through these links
guinealynx.info/healthycavy.h…
guinealynx.info/diet.html
guinealynx.info/scurvy.html

She needs to go to the vet because the symptoms you describe could be scurvy but they could be lots of other things as well. in any case, make sure she is getting plenty of vitamin C. There is no excuse for a pig getting scurvy with the availability of guinea pig chow fortified with C, crystalline vitamin C to add to the water and the year round availability of fresh produce

Losing weight isn't really a sign of scurvy. Neither is a funny walk. I think you need to see a vet and see if there's a problem.

Give her Vitamin C.

The good thing about scurvy is that reverses quickly.

Cut up an orange.
Get a seperate water bottle and put in some fortified fruit juice in it.
Get some powdered vitamins to put on her food.