Really wierd braking

[Yii]

I was jut dropping off my Gf to school today and we approached a stop sign and when i slowed down the brake just sounded really really weird. Slight vibration and kinda felt like my brake wasnt doing its job as well as it suppose to. I pulled over got my flash light out and checked my brakes and everything was okay. There was a good amount of the brake pad left and nothing that seemed out of place. i am going to bring this to a shop soon and get it checked but i was wondering if anyone can tell if this is a warp rotor or a sign of a warped rotor? it went away right after that single incident aswell and now everything is fine. Traveling speed was about 20 to 30 mph not fast at all to a complete stop.