She was very energetic before lunch, but she felt sick afterward.
Literal
Lunch-before [contrast-は] very well was [but-が] after-that feeling [subject-が] bad became.
The は after 昼食前 is contrastive — 'before lunch (specifically), she was fine' — setting up the contrast with what came after. が connects the two opposing states. 気分が悪くなった uses ~なる ('to become') with the negative adjective 悪い to express a change of state: from fine to feeling sick.