The word commonly used by Australians is pom (or pommie). The convict shirt-marking is historically true but like many words of today, the spelling and meaning has altered. Aussies today generally refer to us POMS as British (particularly English) and use the reference of Prisoner of Mother England to explain it. It's their word and that's the way all of them explained it to me years ago!
If the pain is easing after you get up then it may not be serious. However it's worth checking out in hospital or a good clinic, just in case. A good doctor can tell you if you should have an x-ray to help his diagnosis, which is not expensive.