Switched string gauge on my Hollow-Body and buzz sound is coming from upper neck?

I just bought an Epiphone WildKat Hollow-Body. I wasn't too sure of the gauge that was already put on, but it was smaller than a 10. I switched them because I prefer the 10 gauge DR and after I put them on, the upper neck started to make a buzzing sound when I played a fret. Mainly on the E and A. Could it be the Nut? It looks rather tight on the up there. Or what?