It Is Hormones

Have the hormones tested.  Then, supplement whatever hormones are low.  I would get natural cat hormones if available, otherwise your cat can get cancer.  Spaying or neutering an animal is no different than doing it to a person.  And just doing a tubal on a female, person or animal, often leads to  PTLS.  When animals get spayed or neutered (especially when very young), they often experience skin problems, which are hormone related (and probably many other ailments they cannot tell us about).  When done young (under a year), it can lead to underdeveloped organs and they can have symptoms associated with that.  Also, it wouldn't hurt to run a full metabolic panel to make sure the thyroid and the rest of the system is functioning ok.