The most common reason is that you have just qualified for a PetCLUB voucher which will reset your points. You can check if you have a voucher by logging in to your online account on the Jollyes website or by opening the Jollyes PetCLUB app. Your new voucher will appear under the voucher heading of your account.
Alternatively, some of your Pet Club points may have expired if you have some transactions older than 18 months before you have been able to qualify for your PetCLUB voucher.