Sorry that is going on. Doesn't sound right, does it? Reasons could be that your hormones have gotten out of whack. Or you have fibroids which can cause that type of bleeding. Or you have a ovarian cyst (common, not dangerous) and it is passing. Do you have any cramping as well?
Unfortunately, I do think you need to see your doctor. But there are places like 'planned parenthood' if you live in the states that are really reasonably priced for women to receive gynecological health care. Otherwise, go to the doctor that prescribed your birth control.
I hate to tell you what you don't want to hear but particularly if it's heavy bleeding you can easily become anemic. That brings a host of other problems. There are a number of things that could be causing the bleeding from missed pills to endometriosis or polycystic ovarian syndrome. If you're sexually active it could be related to an injury sustained during intercourse. It doesn't have to be something traumatic to cause something like this. again I don't know anything about you just throwing things out there. Could be an std or some other type of infection. I'm also not sure that if you're in active miscarriage that a pregnancy test would say that you're pregnant. I think for your best interest you should seek medical treatment. Lots of the potential causes if left untreated could lead to complications with your ability to get pregnant in the future or to carry to term. Good luck. Better safe than sorry.