Big bump on my male cat

My car simba isaround 4 months old. We got him fixed a few weeks ago, and now he has a big bump right before his penis. I didnt notice it until i gave him a bath today. Its squishy, i am overly worried but we went to the vet thursday to get his shots, but they said since he wasnt eating theyre gonna put him on medication. Well he has been eating and still playing acting normal. But lays around a little more than usual, like hes worn out. But he plays hard, with his litter mate and my two smaller kids.
I am worried because i just had a cat come in the other day with fip and his isnt as bad as her stomqch but thats what the vet brought up too. Just we just spent alot of money for everything when we went to the vet thursday. What else could it be?

How big is the bump?  The vet has not seen this lump, correct?  What medication was he put on for not eating?  Do you have any idea why he wasn't eating?

Its not big, well nala my other cat. Had respiatory spray, she got put on medication for it. Right when they got payed and neutured, and shes all better. But he started showing signs of it, so when i took then to get there shots this last thursday. And he went on it and didnt get his shots. She said since he wasnt eating she couldnt give him his shots and see if he gets better by next thursday. Then i can get his shots done, but if it doesnt work. It could be fip. But hes playing, eating, pooping fine. But i gave him a bath today and saw it. Its right above the penis, so im confused. Because its bot his belly, but hes been laying around alot.

Oh, and it goes away in different positions he has

I would see if you could send the vet a photo.  If he's already going back this week maybe try to move up the appointment.  It sounds like he's well enough for shots.

How big is this bump though?  Can you compare the size to something?

Is it actually on his penis like when his penis comes out can you see it or is it on his scrotum? If it is then I think you should take him to the vet so they can see what it is!

Though I can't get rid of the niggling feeling that maybe you're describing the primordial pouch? (is it spelt like that omg idk?) I'm not sure why cats have it but I found one of my rescues, Cipher, has a very prominent one (I found it when I gave him a bath, too!). It's kind of between their back legs and feels like a bag of water I guess? Could it be that? Though I still think you should go to a vet to be sure!