"Ah… I haven't been feeling well…" "Oh. Are you okay?" "Hehe. Actually, I'm just playing hooky."
Literal
Ah... a-bit, body-condition [subject-が] not-good-[and-て]... / That is [explanation-なんだ]. All-right [question-か]? / Hehe. In-truth, skip-rest is [explanation-んです].
体調が優れない is a set polite expression for feeling unwell — 優れる ('to excel, to be superior') in negative form means 'not to be at one's best.' Formal but still spoken. ズル休み is a compound of ズル ('cheating, unfair') + 休み ('rest, day off'), meaning 'skipping (school/work) without a legitimate reason, playing hooky.' The contrast between the polite phrasing 体調が優れなくて and the shameless confession ズル休みなんです creates the comedic punch.