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認爲，如果你的寵物出現跛行或不願意使用它的一條腿，並且隨着時間的推移越來越嚴重或沒有改善，而且沒有明顯的原因，請儘早去看獸醫，因爲它可能是癌症。