There are a number of reasons why PSA can change and that is why it is not a reliable indicator, particularly early on and without a history of tests. You have been diagnosed with 3+3, presumably following biopsy, so your cells are a little different to normal cells. As to your seemingly logical question about how can I man have a very low PSA but still have cancer, you have to accept that it can happen, particularly with sone types of PCa. (There are quite a number of types). Interestingly, a well known doctor from Mayo, one of the leading USA hospitals, put up some slides in a talk showing a man with zero PSA but with extensive advanced mets, although this is quite rare. It does indicate how PSA can sometimes give the wrong impression. Incidentally, the opposite can also apply where PSA is high but cancer cannot be found.