Verb has 2 senses

• act up(v = verb.social) carry on - misbehave badly; act in a silly or improper way; "The children acted up when they were not bored"

Sample sentence:
Somebody ----s

• act up(v = verb.perception) - make itself felt as a recurring pain; "My arthritis is acting up again"

Sample sentences:
Somebody's (body part) ----s
Something ----s