Hi Puggirl, welcome to Koko's, we're getting quite a few Aussies on here lately and it's good to see B)
20L is around 5 gallons so I'm sorry to tell you that you're tank is not large enough for goldfish unless they are tiny weeny babies (this is because gf eat alot and make a lot of mess). Nevertheless, you can make do until you move and get a larger tank but it'll be quite a bit of work.
New tanks are very dangerous, even when they are large enough for goldfish. Until your filter has cycled which can take 4-6 weeks your water can quickly become toxic and even kill your fish. This is how it happens :-
Fish eat and breath then poop and pee creating waste products. About 3 days into the life of a new tank ammonia will become present in the water as a result of these waste products. This is deadly, change a little bit of water everyday and purchase some water conditioner that neutralises ammonia, such as Prime or Amquel or Stress Coat, there are lots on the market. I prefer Prime.
As your tank matures (around day 5) bacteria will grow that transform the ammonia into nitrites, guess what? these are deadly too.

More daily water changes (Prime actually neutralises nitrites as well).
Eventually around day 21, bacteria will grown that transforms the nitrites into NitrAtes, these can be keep under control by weekly water changes of 30-50%.
Look on Koko's home page for more detailed information on "The Cycle of the Tank". Some members, including myself, use a product called 'New Improved Cycle' that adds the bacteria you need to the water and helps your tank cycle more quickly.
Now to your unhappy fish. All the symptoms you mentioned sometimes do occur when fish move to a new home, some fish stress out more than others. What worries me most is the spitting out of food, that's not good sign. It could mean an internal bacterial infection.
First of all get your water in order. If you don't have any water conditioner get some ASAP. If you can afford it buy some test kits for ammonia and nitrite, or take a sample of your water to the lfs (local fish shop) and ask them to test it for you. Most shops do it for free.
If your fish is still not eating or improving in 2 days, post back and someone will recommend medication.
Good Luck!