Hey Natasha, welcome to the forum, but no, no, no. While amoxicillin IS used on cats, it's impossible to say how much of a 250mg capsule to give since kitty is much smaller than we are. And besides, you don't know What the lump is or even if it would respond to an antibiotic. Just as a good rule of thumb, never give human meds to kitty.
Please give your vet a call on this one.
Natasha, I have to say.....NO WAY!! First of all, you have no idea whether that lump is an infection or not, and secondly, giving human meds is a bad idea unless you get the okay from your vet. That 250 mg capsule is for an adult human being that's over 100 lbs. How much does your cat weigh??????
Please, just take your cat to the vet. Some human meds are deadly to cats!