My first ultrasound was 19 dpo, the same day as my hcg levels. They could see the sack in the uterus at that stage, but not much else. Once they ruled out an ectopic, they had me come back at 8 weeks to check for the heartbeat. My guess is with your history, they want to make sure it has implanted in the proper place.
Hang in there. The emotions are a crazy roller coaster ride. I had full blown anxiety/panic/crying jags the night before every ultrasound until after 20 weeks. I still get a little nervous and I am getting them every two weeks and can feel him move daily at this point. PAL is a little quiet, but you can ALWAYS PM me if you need to talk. I tend to be on JM through out the day and most evenings, so it won't ever be too long before you get a response.
Kat.