When the adverbial particle か is combined with と, the う-Verb 思（おも）う, and either of the hypothetical markers たら or と, it forms a compound that is used to illustrate uncertainty about whether (A) has been finalized or not. In most cases, this translates as ‘(A), then again (B)’, ‘just when I thought (A), (B)’, or ‘no sooner than (A), (B)’. Literally, the meaning is close to ‘before (A) could even be confirmed as having happened, (B)’.
かと思（おも）うと and かと思（おも）ったら will regularly appear after verbs in either their dictionary or past-tense forms.

• 何（なに）を言（い）い出（だ）すかと思（おも）ったら、みんなの前（まえ）で俺（おれ）の悪口（わるぐち）を言（い）い出（だ）した。
  Just when I thought of something to say, he started bad-mouthing me in front of everyone.
• 赤（あか）ちゃんが泣（な）き止（や）んだかと思（おも）ったら、また大声（おおごえ）で泣（な）き始（はじ）めた。
  Just when I thought the baby had stopped crying, he started to cry loudly again.
• 娘（むすめ）が宿題（しゅくだい）を始（はじ）めるのかと思（おも）うと、パソコンを開（ひら）いてユーチューブを見（み）始（はじ）めた。
  Just when I thought my daughter was going to start doing her homework, she opened the computer and started watching YouTube.
• 子供（こども）が転（ころ）んだかと思（おも）うと、立（た）ち上（あ）がって走（はし）り出（だ）した。
  Just when I thought my kid fell down, she got up and started running.

Alternatively, this structure may also be seen following nouns, especially when the meaning is focused on the speaker's feelings about (A).

• 田中（たなか）さんは冷（つめ）たい人（ひと）かと思（おも）ったら、ただ人見知（ひとみし）りなだけだった。
  No sooner than thinking that Tanaka-san is a cold person, I found out that he was just shy.
• 今日（きょう）の試合（しあい）が最後（さいご）かと思（おも）うと、悲（かな）しくなる。
  Just when I remembered that today’s game is the last one, I got sad.

Either なの (or の when used with verbs) may appear between the target phrase and かと思（おも）ったら when the (B) phrase includes the speaker’s feelings.