Was is wrong of me to stare at another girl when I was wih my girlfriend?

i have been dating my girlfriend a few month and I'm in love with her, but there's a girl I usually check out in school she's a couple years younger. and she's got really big . I usually check her out on the brakes and she checks me out too. we haven't spoken or anything but we always check out each other. we were at a concert and this girl was standing behind me and I had my hand round my girlfriend and her head was resting on me so she couldn't see me staring at this to her girl, but I felt kinda guilty since I was staring at this other girl so much, what does this mean and was it wrong of me?