Did I have a miscarriage?

I'm hoping someone may have had a similar experience or some input for me. During the month of January I my period was about 2 weeks late. It lasted from January 28 until about January 31. Usually when I first get it, the flow is very light and gradually gets heavier. This time though, it was so heavy I didn't even have a chance to get to the bathroom. As gross as this sounds, I felt/saw a clump of blood fall into the toilet. Before getting my period I had felt nauseous and slightly lightheaded, back pains, and just not feeling well. About 2 days after the bleeding stopped I was lightheaded, nauseous(almost vomited), in a cold sweat, blood pressure dropped very low, and passed out. I actually passed out twice that day. The next couple days I felt dizzy and nauseous and sleepy and just not right. When I had taken pregnancy tests around the time of my missed period, they came back negative, but I know they aren't always 100% accurate. I went to my doctor because of the way I had been feeling and he gave me a list of things that could be wrong, one of them being a miscarriage. Because of the negative pregnancy tests I said no, definitely not. But after thinking about it, a miscarriage doesn't seem too far fetched. Does this seem like a miscarriage? If so, is there anything I should do medical wise? Is there any way to find out definitely?Thank you for any help!