:welcome: Hey Rainel, welcome to the forum!
It's fantastic that you're doing some research about goldfish care. Hopefully we can help you out and not scare you off!
As other members have already stated, you need to look into a larger tank for your goldfish. It's important that you find her a larger tank soon, even a 10 gallon or 20 gallon, since she is so young.
The environment in which a goldfish lives for the first year of his or her life can have lasting effects throughout its 20 to 50 year lifespan. Goldfish do much of their growing in that first year and being that cramped for even a month or two can trigger diseases and infections that can cause problems in years to come.
In the long run, you're right to look at 50 gallon and larger tanks. One sarasa would be comfortable in a 70 or 75 gallon tank for its entire life, but there's nothing wrong with starting smaller and working up to that as the fish grows.
Now then. The symptoms you describe could be a result of many things... the most probable is ulcer disease, which is caused by poor water conditions, overcrowding, and inadequate maintenance. Although you are doing large water changes, depending on your filter type, maintenance, and feeding regimen, harmful chemicals can build up to toxic levels within even one day in an aquarium that small.
If could also be tumors, lymphocitis, or fish TB. Tumors are fairly common in goldfish, but usually don't 'burst' as you described.
If you can give us a little more information about your water parameters - which you can test with any good test kit available from your pet store - your feeding regimen, and perhaps a clear photo of the fish, I can try to confirm whether or not it is ulcer disease. Also - are you using any dechlorinator on your water when you do water changes?
Good luck and keep us updated! Once we identify the illness, we can talk about appropriate treatment.