I went to the er the 16th they told me I was pregnant but my HCG levels are extremely low so the doctor said either I’m very VERY early or I’m losing the baby.

I went to the OBGYN yesterday the 19th he wanted to do another blood test. He said they should have doubled since the 16th he told me that if they was lower or the same he was going to do a ultrasound Monday to confirm a miscarriage. Well later on they called me with the good new that my HCG level have double to where there suppose to be. So they have made me a appointment for next Friday to see if we can see the baby and find out how long I am. They are saying that I’m about 4 weeks 4-5 days. So my question is can my HCG level still get higher but already lost baby? Also can they drop within the next week before ultrasound? Is there normally a low HCG level right before loss of pregnancy or can it still build up ?