Herpes can remain dormant for years after initial contact, some don't even know when they were infected. Or, she was exposed to the virus just before the baby was born. In my case, my husband infected me just after the birth of our son. Symptoms showed up within ten days.
My sister, however, had an allergic reaction to colored toilet tissue resulting in a similar rash. Thankfully in her case it was just that, not herpes. Please let me know if I can help with any other questions.