After giving birth, my cat has a hard lump. Is it another kitten?

Congratulations on the kittens. If Fluffu did not have an xray to determine the amount of kittens prior to labor (which I assume based on your question that she did not) then it is hard to say for certain about what you are feeling. The "hard lump" could be her uterus not yet contracted down to a small size, her mammary glands (those are outside the abdomen but you may still be feeling them as the "hard lump"), another kitten, or simply a full stomach. Hopefully there is not another kitten in there at this point since she is finished with labor. You can monitor and watch her closely and continue to feel the "hard lump", but if you are pretty suspicious about the possibility of another kitten the safest thing to do is to have a veterinarian examine her. The doctor may be able to tell just on physical exam what the hard lump is or can take an xray for definitive diagnosis of knowing if there is another kitten in there. This would be the best and safest thing to do for Fluffu. Thank you for reaching out and best of luck to the momma and her new babies!