What do I make of this?

I had a sudden surge of anxiety and emotion while driving home today and I don't know what to do about it. I suddenly felt extremely stressed about something, then burst into tears, almost hysterical tears, and felt like something awful was about to happen, or already had happened. I'm sure it wasn't really a premonition or anything, but I still feel sick to my stomach and nervous. I have no idea what I could be nervous about. I'm not particularly stressed about anything...Any ideas on what just happened?