What's the matter with my cats leg?

Yesterday morning my cat came in after being out and she came in limping, she was limping on her back leg all day yesterday and when her 5 week old kittens tried to play with her, they sometimes accidently jumped on her leg and she hissed and bit them quite nastily. After waking up this morning too see if it had got any better, it has now swelled a considerable amount. I have booked a appointment for the vets tomorrow and i am deffintly taking her in to see whats the matter with her i was just woundering if anyone has any idea what it could be as i can't wait until tomrrow to see what could be wrong with my girl?