Why would your ex-boyfriend hide from you the fact that he has a new girlfriend?

Because he wants it all. He wants his new girlfriend and do as he pleases (have fun) but doesn't want you too. Could it be love? Possibly. I doubt he's sure where he's at so he wants his cake and eat it too. I think you're pretty smart, so get on with life and leave this guy in the dust! If he cares enough for you he'll come back and if not then you just saved yourself a whole lot of loss and heartbreak in your life. Good luck