Paulo, was there definitely not a < sign on the results? Have you asked for a print out? Our GP practice reception staff apparently have no idea what the 'less than' sign means or how important it is in relation to blood tests so John asks for a print out each time just to be sure.
Regardless of whether your result is <0.03 or 0.03, you have no reason to be concerned unless your PSA rises above 0.1 and then rises in two further consecutive tests.
Your body may just be settling back into what is normal for you, or the hospital / GP may have changed to a new testing lab, or the lab may have purchased new machines, or the existing machines may have been calibrated since your last test.