I think I might be pregnant, but every test comes back negative. Can someone help me?

I'm 18 years old and I haven't gotten my period in 3 months now. I am on birth control, but my periods have never been irregular before. I have been to the OB/GYN, my regular doctor, and the ER a couple of times. Myself and all of the doctors have done the pee tests as well as the blood tests, and they all keep coming back negative. Lately I have felt extremely tired, having to go to the bathroom more, experienced heartburn and I have never had it before in my life, throwing up every day a couple times a day for the last week, and about 2 weeks ago along with this morning I started bleeding brown blood. I thought that maybe it was my period, but it slowly disappears throughout the day. I have also been getting sharp pains in my pelvic area which I went to the ER for. My aunts HCG levels didn't increase with one of her children, so maybe it's hereditary? I've had an ultrasound and nothing came back on it. I am scheduled to go for another one on Monday. Anyone have any ideas?