If it was Velvet, it doesn't have to be all over them. It is still a parasite and since she had just spawned and given birth, she was in a state of stress which would make her more vulnerable. It could also me that the tank may not be totally cycled after having it only a month. Could you please post the readings on an Ammonia, Nitrite and Nitrate test if you have them? It would help a lot.
If there is Velvet in your tank then you need to raise the temperature Slowly (no more than a degree every couple of hours) to 83 degrees and leave it there for at least 14 days. Also do a 30% water change to get rid of any possible Ammonia, Nitrites, and Nitrates that there may be in case the problem is with the tank cycle process. (before you raise the temperature.)
Please let us know the water testing readings if you could.
Welcome to Fishlore.com. This is a very good place to learn about the fish you care for. It is my hope that I have helped a little and that you enjoy your time on this forum as much as I have.
Rose
