I don't know if I have a bad pillow or not, but I have a really stiff neck.
I don't think I have the right pillow for my neck because my neck always get so sore.
ーI don't know if I have a bad pillow or not, but I have a really stiff neck.
「枕が悪いのか、肩こりがひどい。」
have a really stiff neck で「肩こりがひどい」
ーI don't think I have the right pillow for my neck because my neck always get so sore.
「枕が首に合ってないのか、首がいつも痛い。」
my neck get sore「首が痛くなる」を使っても表現できます。
ご参考まで！
カナダ 
