Soon after an incident, sit down with her. State what you just observed and make clear you raise the issue because it's not a one-time occurence. Give examples of other times her negative behavior has impacted others in the company.
Say, "We need to get to the bottom of it because I think you're acting inappropriately to other people. It affects the team, the quality of work and your own reputation. What do you think causes it? I'm here to helpy ou figure it out. Knowing you, I believe you don't want to negatively affect the team or yourself."
When you ask an employee a question like, "what do you think causes it?" you give her a chance to speak for themselves, without the burden of your assumptions. But you also have to be prepared for her answers!
You need to prepare for the possibility that she says, "I'm sorry, but it's PMS." Thank her then say, "Okay, if you think that might be the source of your behavior, that's fine. So I need you to do whatever you have to do to act appropriately."
If the cause is internal, PMS or not, she has a responsibility to get back in control of her behavior. Tell her to keep an eye on her behavior, and if she senses it's "drifting" she should take steps to get back in control. And ask what you can do if you observe her behavior is affecting the workplace.
Regardless of the cause, stay focused on what amounts to appropriate behavior in the workplace.