It's stressful when someone who I haven't seen in a while asks me, "have you gained weight?"
someone who I haven't seen in a while
久々に会う人
厳密にいうとwhomで正しいですが、ネイティブは大体whoを言いますね。
直訳的なsomeone who I haven't met in a whileも通じますが、seenの方が自然な表現ですね。
～is stressful
～はストレスです
someone who I haven't seen in a while asks me, "have you gained weight?"
久々に会う人に「太った？」と聞かれる
ご参考になれば幸いです。
It's stressful when I see someone I haven't seen for a while and they ask me if I've gained weight.
It stresses me out when someone I haven't seen for a long time asks me if I have put on weight.
ーIt's stressful when I see someone I haven't seen for a while and they ask me if I've gained weight.
「久しぶりに会う人に太ったと聞かれるのはストレスです」
It's stressful ... で「ストレスです」
someone I haven't seen for a while で「久しぶりに会う人」
to gain weight で「太る・体重が増える」
ーIt stresses me out when someone I haven't seen for a long time asks me if I have put on weight.
「久しぶりに会った人に太ったと聞かれるのにストレスを感じる」
It stresses me out ... で「ストレスを感じる」
someone I haven't seen for a long time で「久しぶりに会う人」
to put on weight で「太る・体重が増える」
ご参考まで！
アメリカ合衆国 
カナダ 
