12 day period?

I've always had a pretty normal cycle when it comes to showing up. They do last about 6-7 days and are usually heavy. All normal, it's the way I've always been. Except whenever I tried any type of birth control, then I would have extended bleeding/spotting between periods. I just recently(9 months ago)gave birth to my second child. My period returned about three months after I had her(we were breastfeeding). I've decided to keep myself off of birth control because, for myself, it does more harm than good. Anyway, my last two cycles before the one I'm currently on we're normal in length but one days 5-7 I experienced some severe dizziness upon standing along with some nausea. Now, I'm currently on day 12 of my period. It was normal for about the first seven days and then when I thought my period had ended i just continued to spot. I'm now experiencing some pretty unpleasant cramps. What could I chalk this up to? Has anyone with a normal cycle experienced something like this without any obvious change causing it? I'm crazy paranoid so I've already made a doctors appointment but I'm just looking for some opinions.