ㄴ가 바뀌다

something has changed

뭔가 means 'something, I couldn't say what', and 바뀌다 is changing of its own accord, so 뭔가 바뀌었어 is exactly the English 'something's different' — you sense it without being able to name it. Seven of these lines are 뭔가 바뀌다; the eighth uses 언제부턴가 (at some point, I'm not sure when), the same vagueness attached to a time. Contrast 바꾸다, where somebody deliberately changes the thing.

Register: casual