My kitten has a lump on her belly by the incision and she has had it drained twice. It is just fluid but I am still concerned. I have had two other kittens spayed and never had this problem. What is causing it and should
She be on an antibiotic? She is playing, and eating normal seems fine. The lump is the size of a plum.

If this is just a clear to slightly yellowish fluid, it is a serum pocket, (seroma). It is fluid that collects in subcutaneous spaces below the skin. (Around the surgical site in this case). It is a combination of both lymph and blood serum, arising from the damage to tissues during the procedure performed.

It is not an infection, but could easily develop as the fluid is prime media for growing bacteria. In spay operations, the surgeon must be aware of mammary tissue that may appear in the surgical site. You mentioned your "kitty", which doesn't accurately describe age of your cat. If it is indeed a young kitten of only a few months, this should not be a worry, but if she is over 7 or 8 months, it may be a factor, as this tissue is rich in both blood and lymphatic vessels. Be persistent in follow up visits if this does not resolve itself with periodic drainage or aspirations.

Was it drained by a vet, or you? Has she been back to the vet to have stitches removed, or did they use dissolving sutures?

If you haven't talked to a vet, then you should. If it's an infection, then she needs to be looked at to make sure it's not more serious than it seems, and put on antibiotics. If it's not an infection, then she needs to be looked at to find out what is causing the fluid, find a way to treat it, and prevent it from developing into an infection.