Yeah if you didn't remember water conditioner then that could be it.
Or he could be one of those guys who acts a little funny for the first bit after a change. My one guy, Jasper, is always a little bit of a brat for about six hours after I put him back in the tank after it gets the 20% water change and filter cartridge switch-out because he doesn't enjoy his time spent in "the cup".
Also; is the water heated at all? One of the things to always make sure is that the water temp is the same or very close before you switch the betta over. Otherwise he'll get a bit of a shock.