to feel sick or unwell — used for physical symptoms (nausea, dizziness) or to express feeling unpleasant or upset.
昨日から頭が痛くて、今も気分が悪い。
I've had a headache since yesterday and still feel unwell.
車に酔って気分が悪くなった。
I got motion sickness and felt sick.
彼の失礼な言い方で気分が悪くなった。
His rude remark made me feel bad (uncomfortable).
薬を飲んだら少し気分が良くなった。
After taking the medicine, I felt a little better.