Please help on how to get my legs thinner?

i'm a skinney girl..US size 4 clothes (UK 8) and used to never ever put on weight but all of a sudden im gaining so much weight only on my thighs..my legs are now mive and look so out of proportion to my thin waist and upper body..how do i get them thin again without muscle on them..i used to do athletics for 10yrs so i just want thin thigs and no muscle cause right now they look like mans legs..should i start a diet ??? im so depressed and cant stop thinking about them