YSK that if your pet develops a limp or reluctance to use one of its legs that gets worse or does not improve over time, and there is no obvious cause, go to the vet sooner than later because it may be cancer.
from /r/YouShouldKnow
YSK认为，如果你的宠物出现跛行或不愿意使用它的一条腿，并且随着时间的推移越来越严重或没有改善，而且没有明显的原因，请尽早去看兽医，因为它可能是癌症。