My stomach has been acting up.
→  속이 이상해.

[예문]
※  act up : (기계에) 문제가 생기다,(몸이) 불편하다.

My computer is acting up. I need to get it fixed.
내 컴퓨터가 말썽이야. 수리를 해야겠어.

I don't feel well. My stomach has been acting up.
몸이 안좋아. 속이 이상해.