During that stage their entire back end is going nuts! Some kitties can't distinguish the contractions, reproductive organs and stimulus from bladder control. There is a constant release or necessity of, sometimes peeing is associated with this. Remember that it takes a few months for complete hormonal re balance after spay,the physical effects of "in heat" should not reoccur, but temperament, personality changes take some time. You want to be sure to get rid of any scent left from previous urine marked areas so this doesn't continue, very important to do so after spay so it does not become behavioral.