Yesterday I heard voices like a girl whispering but I couldn't make out what she was saying, was this a panic attack or something worse?

Talk to your doctor. That is a difficult question, and it can't really be answered in a forum like this. I would certainly bring this to the attention of your primary doctor as soon as possible. If your symptoms start to get severe, or you fear for your safety, head to the emergency room for an immediate evaluation.
Panic w/ voices. Auditory hallucinations are not common inpanic disorder. Best to talk to your doctor to review your medications, evaluate your health status, & rule out other factors such as alcohol or drug effects.