Miscarriage..?

I'm 16. I was late about 4 months for my period. At first I didn't think much of it because my periods are usually really irregular anyways. But this time, something felt off. I took two pregnancy tests. One said positive and the other said negative. I never got the chance to get a third test to see which one I truly was because I then randomly and very heavily started my period. Well, a pink bean looking type thing came out and I immediately became concerned. Either, I was not pregnant and I just started my period, or I was pregnant and I miscarried. I told my boyfriend and my parents about my pregnancy concern from the very beginning and I have a doctors appointment on Thursday. Bottom line, what are some symptoms of a miscarriage? And why would I have one?