I'm sorry. I'm sending you virtual hugs.
I don't remember what it's called, but I was watching a medical show on discovery and I also remember symptoms of pregnancy possibly occurring when someone is only under the impression that they are pregnant. Maybe there is a mental component that you could research.
On a personal level of advice, I have had similar fears to your girlfriend in the past. It's good that you keep supporting her. My period is often irregular also, but I have often become afraid that someone has gotten me pregnant even before my period should arrive. When I was pregnant I think I started to get used to what would change and what I needed to do. Although I still jump to the conclusion that I am pregnant sometimes my fear is not as bad so I am more patient about waiting a month to find out more easily.
Maybe if you work together to form a plan of action, thoroughly detailing everything down to precise dollars and possible sickness, etc., that you would do in the event of pregnancy it might help her calm down?
I hope this helps.