HCG rises after falling drastically!

My HCG levels were 2420, 5500, 900, now 9550. I went to ER when I started bleeding over two weeks ago. Doctor said it was most likely a miscarriage and follow up with HCG which I did and it went to the 900. Today it is at 9550 and had a emergency Ultrasound. There is a empty sac measuring 7 weeks. I have to get tested tomorrow again. If the levels keep rising I have to get tested everyday a and do ultrasounds otherwise I am going in for a DNC. The doctor thinks I was pregnant with twins and lost 1. The second one no idea...if the sac is empty. Anyone experience this?! I thought this nightmare as over then this just happened! So confused!