I don't know if they have shrunk from washing them too much or the shape of my body has changed, but the pants are tight around my bum now.
I'm not sure if I have washed them too many times or I have gained weight, but these pants give me a wedgie.
ーI don't know if they have shrunk from washing them too much or the shape of my body has changed, but the pants are tight around my bum now.
「洗いすぎで縮んだのか、私の体型が変わったのかわからないけど、パンツのお尻まわりがきつくなった。」
shrunk は shrink「縮む」の過去形または過去分詞形
shape of one's body で「体型」
「お尻に食い込む」は the pants are tight around my bum のように言えるかと思います。
ーI'm not sure if I have washed them too many times or I have gained weight, but these pants give me a wedgie now.
「洗いすぎたのか、太ったのかわからないけど、このパンツはお尻に食い込むようになった。」
wedgie はパンツがお尻に挟まって食い込んだ状態のことを言い、these pants give me a wedgie で「このパンツはお尻に食い込む」と言えます。
ご参考まで！
カナダ 
