Cat back from vets query?

Hi guys,

My cat has just been to the vets to have his teeth cleaned. Before I
took him in his sister went in for the same treatment. When she
returned he started to hiss at her at first - but after a day or so he
seemed to become really depressed and hardly ate anything. He looked
really unwell - and I honestly thought he was at deaths door! I
thought he looked puffy around his sides (kind of fatter) so I took
him to the vets the next day. The vet examined him and took bloods and
said everything was in order. He had no infections etc. He didn't seem
too bothered about his 'puffy sides'. He kept him in for 24 hour
observation and the next day did the teeth.

My cat is back tonight and seems ok - he ate as soon as he got home -
but now his sister is hissing at him!!! Oh joy!

My question is - what might be causing this puffy / swolleness around
his mid-riff? The vet seemed to dismiss it but I feel it wasn't there
before.

Also the bloods did show a higher platelet count - which the vet said
'can indicate cancer - but also many other things' and he said not to
panic and there was no other indication of cancer etc.

I guess I'm just worrying....

Also is it normal for cats to get depressed when another one has been
away to the vet?