治りつつあったのにまた再発って英語でなんて言うの？

ーShe was starting to get better, but she had a relapse recently.
「彼女は治りつつあったが、最近再発した」

ーShe was getting better, but she got bad again.
「彼女は治りつつあったが、また悪くなった」

例：
My sister had an eating disorder for several years, and she was starting to get better, but she had a relapse recently.
「姉はここ数年、摂食障害を患っていて、治りつつあったが最近再発した」
eating disorder で「摂食障害」

ご参考まで！

• "(They) were getting better but (they) relapsed."
  "(They) were getting better" 「治りつつあった」"they were recovering" や "they were in the process of getting better" も使えます。
  "but" 「のに」
  "(they) relapsed" 「また再発」"they got sick again." も言えます。

この場合には "they" は "sister" 「姉」のことを表します。
"My sister was getting better but she relapsed."