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.