Having been a teenage girl, who, at one point, hung around with other teenage girls (albeit ackwardly), I can tell you for certain that they would have laughed and giggled no matter who or what you were.
If you are a gorgeous hunk of burning love, they'll giggle at that. If you're kind of goofy-looking, they'll giggle at that. If you are so ordinary that there is nothing to giggle about...they'll giggle about that.
You see, before you even came into view, they were already talking about sex. They were giggling to each other about that cute guy in Algebra class, and sex, and the English teacher with the armpit stains, and sex, omg, how gross! Tee hee hee. And then you came into view, while they were talking about those things. And now, all of a sudden, TEE HEE HEE!! !! OMG!! ! TEE HEE HEE!!!There's an actual person (19 year old male?) who witnessed us talking about guys and sex!!
So, really. Don't take it personally. It's them. Teenage girls have there little secret unspoken language between them, and all you need to do is come into their field of vision at the wrong time, and you'll be giggled at.