Sorry, but I have to ask. Are you black?

You're either lying when you said you didn't swerve or the cop just saw something such as, and just said you swerved for a valid reason to pull you over:
1) A lot of cigarette smoke. If the windows were up and a couple of you were smoking, a police officer might go after you in hopes you do have marijuana or beer in the car. Teens plus cigarette smoke may just entice a very bored cop to pull you over. Driving a shitty car doesn't help this matter, but I've seen it happen most prevalently in areas with a bunch of hicks.
2) Stoner look, loud music, no muffler. If you all have greasy long hair, 3 people in a car, possibly no shirts, etc, and again, one of you smoking anything, may entice a cop to pull you over.
3) You're black
4) You're a dumbass and really did do something stupid to get pulled over.
4) MAYBE the cop's eyes played tricks on him.

But anyways, you have absolutely nothing to be worried about. My friend got caught with possession and paraphanelia a week or two ago. He frisked us and found only stuff on him, he immediately let me go. You and you friends will get off, but the driver will get a license suspension and around a 150 or 200 dollar fine. Judge and cop have no evidence or even anything to lean on in hopes of booking you for possession.