cat developed lump

my cat within the last week or two had a fairly large bump come up on his side. he is roughly 4-5 months old. hes been taken to the vet and given all his shots. tested for feline leukemia and didnt have it. when we took him to the vet they took a sample from the lump but said the results were inconclusive and they couldnt see anything wrong with it.they told us to wait 3 weeks and if it doesnt go away or if it gets bigger bring him back. they thought it may of been a reaction from his last shots which were 3 weeks prior to the last visit. about a week or two before the bump showed up. it is a hard bump not attached below the skin as you can pull it away. it doesnt appear to hurt him or bother him at all. its almost oval shaped and about the size of two dimes placed side by side. hes still a kitten so it scares me i was just wondering if anyone had any advice or thoughts as to what it could be/ what i should do.

thank you

Where on his body is it? Most shots now are given in the leg--is that where the lump is? I would just keep an eye on it for now, measuring it every 3-4 days to see if it gets bigger. If it does, take the kitty back to the vet right away.

my cat Thor had a large lump a week or two after being vaccinated, it was near the injection site and so hard the vet was unable to aspirate it. He suggested waiting several weeks which I did and it did go away.

Only your vet can really answer that, if you are really worried they can remove the lump and biopsy it and let you know exactly what it was made up of.

on his body its right on his side like over top of his ribs.. right behind his front leg.. i know i could get it removed and the biopsy and if it doesnt go away i will.. but for now the vet said to wait it out his next appointment is in 2 and a half weeks

It sounds to me like a cyst, or maybe a lipoma (benign fatty tumor). I'd just wait it out unless it seems to be growing or bothering him.