〜せいで／〜せいか

Meaning

Expresses that there is an undesirable cause or responsibility behind a result. "〜せいで" identifies the cause specifically with a tone of blame or regret, while "〜せいか" expresses that, though not certain, that may be the cause, with a tone of speculation.

Examples

寝不足のせいで、一日中頭が痛かった Because of lack of sleep, I had a headache all day.
年のせいか、最近忘れっぽくなった Maybe because of my age, I've become forgetful lately.