Hi Imsmith about 3mnths ago I also had a gold female guppy develop similar looking spots on its body after giving birth and it died within 24hrs. Like you I wondered about ammonia burns however the water conditions were normal with ammonia at 0. I did not pay for a postmortem so it is still a mystery to me and I'm sorry that I am unable to help you with the cause.
However if its any consolation I have not yet had any further incidences of this occurring in the tank. So I hope that this will be a one off for you too.
