If someone is an expert on RP-HPLC analysis of urine extracts, they may weigh in with an opinion shortly, or eventually. Your sample prep seems a little atypical to me -- although it can just be a simpler prep that your purposes allows. Often, there is some sort of selective extraction with a solvent to remove things that aren't of interest and concentrate things that are.. So you can look into those sorts o methods. Now, you've got a monster peak in the physiological sample -- all you know is that its more non-polar. If you could extract the spectrum from a PDA detector, we would have a hint. If you could run it on an LC-MS, we'd have a better hint, or maybe the answer. But as it stands now, I don't have a clue.
Another thing you can try is spiking some standard into your sample before you begin sample processing , to see if the creatine is changing . But that seems very unlikely given your simple sample prep. Still, its good advice to do that when you want to make a comparison.