from your symptoms that you have just mentioned you have a boil in your belly area the swelling is just an accumulation of the pus.
you need a course of antibiotics to reduce the infection and ripen the boil fully to release the pus accumulated.
if the swelling persists then you need to be done incision and drainage
to fully remove the pus.
it is a normal disease and not a strange disease it is usually brought about if your hygiene is not good, worsened by heat condition, if another person had it and you by mistake touched the fluid you could have gotten it.
hope have answered your query
get well soon.