Miscarrying a twin ?

I am 29 years old and Have four children. My last pregnancy was identical twin boys. I have two daughters that are 2 years apart. I have had a miscarriage and a D & E before my twins. Last week I discovered I was about a week late for my pd, and was having pregnancy symptoms. I pretty much know my body so I decided to take a pregnancy test at home. It was positive. I took a second the next morning to be sure and it was positive as well. But later on that night I began to spot a brown discharge. I figured it was implantation bleeding until it started to turn pinkish and then soon red. I went to the emergency room only because this happend with my twins, I bled badly for three weeks with them and they turned out to be a normal healthy pregnancy. I never did the HCG beta trend with them. Anyway (i'm trying to keep this short), When I went to the emergency room ( 1/9) they told me that the OS was closed, beta was 1100 and ultrasound confirmed a gestational sac at about 4 weeks old. No fetal pole yet, but they said it was too early for that. So I followed up with my OB (1/11) and she did another Quant. Count was 1,984 and I had another one on 1/13. Also another ultrasound confirming a normal healthey 5 week gestational sac. However my beta on 1/13 went down to 1,936. I would normally think i was miscarrying if it wasn't for the positive ultrasound news. So my question is, is it possible that I might have miscarried a twin and that's the reason for the slight dip? I dont know what to think. Anybody have some insight. I was also told that if I miscarried, the ultrasound would not have turned out the way it did. Advice, Please:) Good or bad.