Effexor messes with your serotonin and norepinephrine levels (neurotransmitters in the brain). I have a temper, but I knew I was experiencing Effexor withdrawal symptoms when I would go from mildly annoyed/irritated to I-want-to-throttle-you/homicidal maniac in the blink of an eye. While reducing and after getting off Effexor, it takes your brain a while to figure out how to rebalance serotonin and norepinephrine.

P.S. You were out of town for a few days? Could have been a revenge poop and barf! But cats can be very finicky about litter and whether, or not the catbox is "clean" enough. Hairballs seem related to age--our cats seem to start having these after years four to six.

On the other hand, our pets are often very empathetic--your cat may be mirroring the distress/stress you're feeling right now as you're reducing Effexor.