Miscarried a month ago...just found out one of my babies stayed with me =)?

I just got back from the hospital...a month ago i miscarried..bled heavily for over a week a lots of large clots...My last u/s was 3 weeks ago and at the time a small oval sac was present but i was told it was just fluid..Got a phonecall today as the had detected something while going through my 3 week old ultrasound...had a another scan and a heartbeat was found...now yesterday afternoon i gt a negative preg test..then find out today there was a heartbeat..im very emotional atm kos now i know i was carrying twins and didnt even know...has anyone had this experience before? i am still shocked xoxox... although i know on my first ultrasound my gestational sac did measure larger...and i had a feeling when my stomach continued to grow...i am 9 weeks pregnant with my twin that survived...still dont understand why i got a negative on a hpt...happy and shocked...