2003TL-S

I just noticed something weird about my car when I accelerate in first gear. When I go full throttle from 1st gear it seems ok until around the point where VTEC kicks in, around 4800rpm. It kind of hangs very slightly for not even a second and then starts going again. I haven't had this problem before, and it has been only happening since the last time I filled up, which was right after I raced a previous gen M3 (96 or 97), which I am proud to say I beat. Coincidence or could it be bad gas?? Any ideas??