After two weeks in the hospital, my acne decreased dramatically.

Literal

Hospitalization 2-weeks did-[conditional-たら] acne [subject-が] drastically decreased.

入院2週間したら 'after being hospitalized for two weeks' — the たら conditional here marks a temporal sequence: 'once I had been in hospital for two weeks.' ニキビ 'acne/pimples.' 急激に 'drastically, rapidly.' An unexpected health benefit of hospitalization.