Siamese Cat behavior change

Question
I have two Siamese cats, almost 6 years old.  We got them together, and they are most likely brothers.  They have always been very friendly towards each other, sleeping together, grooming, etc.  They have always played pretty hard together (wrestling, chasing, etc.)

About a week ago, our blue point had a sudden change in attitude towards the other.  He started hissing, growling, spitting when the seal point comes towards him.  Not all the time, but often.  They will still sleep together sometimes, but their relationship is definately changed.  The seal point doesn't seem to afraid of the hissing, and will still try to wrestle and chase.  I hesitate to use the word aggressive, because it really seems like our blue point is very defensive.  

The day it started, we had a vet appt. already scheduled (yearly checkup/shots).  The vet saw nothing wrong, but there really wasn't a full examination.  

Any ideas as to why he had such a sudden shift of behaviour?

Answer
Jason, did this sudden behavior change before or after the vet visit?  If after, the traumatic experience of the car ride, vet visit and the unfamiliar smells the cats carried home could very well be responsible.  Given time, cats will usually relax and forget the experience.

However, any change in behavior does warrant a full exam.  Things like a belly ache or dental disease (very common in Siamese) can cause a cat to be irritable.  A vet can usually tell if a cat is in pain by palpating him.  The cat tenses up when touched in a painful area.

If he gets a clean bill of health, might it be possible that this is redirected fear aggression?  It's relatively common for a cat to redirect aggression to an innocent bystander if he experiences something that scares him.  This could be anything from seeing a strange cat through a window, or a loud noise occuring while in the same room as the other cat.  If there are cats outside that seem to upset your  kitties, sometimes a blind secured on the windows will keep them out of sight.

For now, I would recommend keeping your two kitties apart.  The reason behind this is that your blue point's defensive behavior may cause your seal point to be fearful, triggering defensive behavior on HIS part.  This results in a snowball effect and continuously worsens the problem.  So keeping them separated for a short time (may be days, may be a couple weeks) is really the best idea.  Allow the blue point to get over whatever has spooked him, and then try reintroducing them every few days until neither seems aggressive.