Stretch marks??

i am a 16 year old girl and about a year ago(when i started dancing) i noticed these 3 little redish-purple lines on one of my inner thighs. i thought nothing of it and just kept dancing and stretching everyday. pretty soon it turned into at least 20 redish-purple lines on both of my thighs!! and now ive been noticing them on my hips and too!! its sooo gross!!! so im pretty sure they are stretch marks. i was wondering if maybe stretching and dancing everyday was what caused them? because thats when i first started noticing them. but then i stopped dancing(not because of the stretch marks) so i gained 10-15 pounds and thats when they got even worse!! so did the weight gain or the dancing cause them? also how do i get rid of them?? i have been using palmers lotion for the past 2months and its not working!!!