〜せいか

A softened version of 〜せいで: the speaker guesses at a cause without being certain. It usually introduces a negative or undesirable result, though it is sometimes used for neutral ones. Often appears with 気のせいか and with sentence-ending expressions like 〜ようだ, and the tone is tentative rather than accusatory.

• 寝不足のせいか、頭が痛い。

  ねぶそくのせいか、あたまがいたい。

  Maybe because I didn't sleep enough, I have a headache.

• 年を取ったせいか、最近よく忘れ物をする。

  としをとったせいか、さいきんよくわすれものをする。

  Perhaps because I've gotten older, I often forget things lately.

• 雨のせいか、今日は店にお客さんが少ない。

  あめのせいか、きょうはみせにおきゃくさんがすくない。

  Maybe because of the rain, there are few customers in the shop today.