If you're someone who bottles emotions rather than feel them out, the effect may come up on you later. If there is a lot going on in your life right now, your thoughts may be at capacity already with what you're already focused on. You might just not feel particularly close to your cat and step-grandmother.
If one of my cats died, I'm sure my wife would be very upset but I'd probably be quite relieved. If it's an unusual reaction for you and you want to talk to someone about it then do! You're the best person to judge if this is odd for you :)