Bleeding while pregnant?

Ok.. First off.. My name is Valerie and I'm 22. I have a 2 1/2 year old daughter and am currently pregnant with my second child. I am about 5 weeks pregnant right now. I have been to the ER 3 times in the past week and to my OBG twice.. I have to go back next week for more blood work and in 2 weeks for another ultrasound.. What's going on is i have been bleeding off and on and spotting every day. They keep diagnosing me with a "Threatened Abortion" which means there's a chance that I could miscarry. With my first pregnancy I had a placenta abruption and had to have an emergency c-section. I don't know if that could have anything to do with the bleeding or not.. The doctors don't seem concerned with it though. I have had 2 l ultrasounds in the past week.. Right now all they see is a sac.. but inside the sac is a yolk sac.. Which I was told was a really good sign.. They said if the sac was just empty that would be bad. I have also had my blood drawn about 3 times.. on Saturday my HCG levels were around 15,000... On Monday they were around 20,000 and yesterday there were 21,000. They said it's a good sign my HCG levels are going up but not a good sign that they are not going up like they should.. They said they should be doubling.. At my next ultrasound they said we should be able to see a baby.. if not.. Then I probably miscarried.. I am just curious if anyone out there has had a similar experience.. and everything turned out fine.. The doctors sound reured about the yolk sac being there.. I am just worried still.. I am so afraid something is wrong.. Any advice or help would be greatly appreciated.. I am also curious about the bleeding being due to endometriosis.. I haven't been diagnosed with it but I haven't been checked for it either and my mother had it.. I'm pretty sure it's heredity so I was gonna bring that up to them at my next appointment. Once again.. Any information or like experiences would be grateful! Thank you!