Cat bitten by something.Area swollenand mushy.Cat now lethargic.

Question
2 days ago my cat came in and hissed when i picked him up.the next morning he had and lump at the base of his tail and he would not wag it and would hiss if i touched it or when i touched his tail.today the lump/bump is about 3 times the size of yesterday and it feels mushy not hard.my son noticed a bite on my cats tail but we cant see any bite where the lump is because of too much hair and his discomfort at being touched.and he has been VERY lethargic the last 12 hours or so.i have some ampicillin alos some keflex.can i give this to my cat/how much and how often?i'm sure the area is another bite and is swollen and full of puss or something.
i cannot take him to animal welfare because due to illness i have been out of work and the refuse to do anything unless i can pay immediately.can you help me,PLEASE?

Answer
if in the uk contact the RSPCA and tell them of your situation.  It needs seeing to by a vet asap as it is probably a cat bite and because cats carry loads of bacteria in their mouths it will turn into an abcess and fill with fluid and puss to try and get the infection out of the body.  Otherwise contact the PDSA or if you are in another country contact your animal protection society (ASPCA) and tell them you need help otherwise if you leave it you could get in trouble for not getting veterinary attention.