Use of 後 in this sentence

Question

「そういうこと考えてたから、俺の方はどうなのかって質問してきたのか？」

「んー、まあそういうこと」

随分とあっけからんとした喋り口だが、その告白がきっかけで色々物思いに耽った後なのだろうということは察せた。

So the character's sister got confessed and she herself is confused on why, so she came to ask her brother for counsel.

I understand

随分とあっけからんとした喋り口だが、その告白がきっかけで色々物思いに耽った

She may speak nonchalantly, but with that confession as an impetuous, she became lost in thoughts,

and

ことは察せた。 is the character own thoughts about how he can guess her situation but how is

後なのだろう here acting here? it's not "after"?

Answer 1

This 後 is just "after", and ～後なのだ is "[it] happens after ～" or "[it] is something happening after ～". The 告白 happened first, then her 物思い happened next, and then this counsel is happening at last. In other words, the confession already caused her to think a lot, and she has already "recovered" now, which is why she's talking nonchalantly now.

Similar examples:

• 8時間寝た後なので、眠くありません。
  I am after 8 hours of sleep, so I am not sleepy.
• 90分戦った後ですよ? 休ませてあげましょう。
  (Now is the time after) they fought for 90 minutes! Give them a rest.