I haven't had my period since two months ago. I went to the doctor and I'm not pregnant. What the hell?

I'm 30 years old. My period isn't clockwork regular, but it usually comes around the same time every month. I have been really hormonal, like PMS, for quite sometime and it's only getting worse! I didn't start or stop a rigorous exercise program, medication, nor have I had an unusual amount of stress in my life. I feel like I really need to start, and everyday I think I'm going to, and this has been going on for about 6 wks!