Meaning of "acting up"

[Lola]

English Grammar Tests, Elementary Level

ESL/EFL Test #88 "Common English Errors (4)", question 2

If my back starts ........., I can't go anywhere with you next week.

(a) acting out
(b) performing
(c) acting up
(d) becoming bad

English Grammar Tests, Elementary Level

ESL/EFL Test #88 "Common English Errors (4)", answer 2

If my back starts acting up, I can't go anywhere with you next week.

Correct answer: (c) acting up

Your answer was: incorrect
If my back starts becoming bad, I can't go anywhere with you next week.
_________________________

please kindly explain to me why is "acting up" here?
what does it mean?

many thanks

Lola

[Mister Micawber]

.
To act up = make itself felt as a recurring pain.
.
_________________
Native English teacher at Mister Micawber's

[Itsybitsyspider]

Hi!

What can we say instead of 'becoming bad'? Is there no such thing as 'becoming bad'?

Thank you.

[Mister Micawber]

'Becoming bad' could work in some sentence, perhaps, but it is not a usual formation. 'Worsening'?
_________________
Native English teacher at Mister Micawber's