You'll get chilly just wearing a T-shirt after your bath. You should put something on top.
You might get cold with just a T-shirt on after your bath. You need to put on another layer.
この場合、次のような言い方ができますよ。
ーYou'll get chilly just wearing a T-shirt after your bath. You should put something on top.
「Tシャツだけだと湯冷めするよ。何か上に着な」
to get chilly after one's bath で「湯冷めする」と言えます。
ーYou might get cold with just a T-shirt on after your bath. You need to put on another layer.
「Tシャツだけだと湯冷めするよ。もう一枚着な」
to get cold after one's bath でも「湯冷めする」と言えます。
ご参考まで！
カナダ 
