Let me ask you some clarifying questions. When did you notice this lump and where is it exactly by the hind leg? On the foot? by the tail? on the belly? Back?
How big and in what frame of time?
What does it feel like? Hard, soft,
Does it seem to bother the rabbit if you touch it?
Is she just in a cage or does she roam ?
What are you trying to say about a hamster?