Had Period 8/28/15 Lasted 6 Days. 9/4/15 Took A Preg

Had period 8/28/15 lasted 6 days. 9/4/15 took a preg test, tested positive. Took preg test next 3 days tested positive. 9/7/15 started to spot, then blood got a bit darker but not heavier. Went to ER, they took blood test, ultrasound and transvagnial ultrasound, found nothinf. Hcg level was at 18. 9/8/15 blood would come out like water only when I urinated, still dark red, flow not heavier. Very small clots would come out. No pain other than very mild cramps. 9/9/15 bleeding started to light up and returned to spotting. 9/10/15 blood was brown, only mild cramps. Today had doc appt, urine test said Hcg levels were 0 and possibly had a miscarriage. This absolutely makes no sense to me. At most I was 4-5 weeks along. Im still nauseous, breasts still tender, urinating very frequently. Could I have really miscarried? Or been pregnant at all??? I'm really heartbroken about this...