I couldn't figure out why, either, with such limited information.
Generally, if an owner who is familiar with their pet notices a dramatic difference from "normal" .... there is *some kind of problem* and depending on the extent of the deviation from 'normal' .... the kitty needs to be taken to a veterinarian for a physical exam and possibly some diagnostic testing.
It is up to you to determine the severity of your kitty's issues:
Is it life-threatening, painful or frightening? Go immediately to an Emergency Vet.
Is it a milder issue? Go to regular vet ASAP during normal business hours.