My cat suddenly hissed at me for no reason. Why?

“No reason” almost always means a reason you didn’t register, not a true absence of one. A hiss can be triggered by touching a spot that’s sore (arthritis and dental pain are common hidden culprits), overstimulation from petting that went on a beat too long, redirected aggression at something she saw or heard that you didn’t, or simple overtiredness from being woken abruptly. A single, isolated hiss from an otherwise normal cat is worth noting rather than panicking over. A new pattern of hissing, especially paired with flinching at touch in one specific area, points toward pain and deserves a proper vet exam rather than being written off as mood.

When to call a vet: A new pattern of unprovoked hissing, especially if it clusters around being touched in one spot, warrants a vet exam to rule out pain.