4 weeks pregnant and questions about Hcg levels.?

I went to the doctor yesterday and got a positive urine test. The doctor did a l ultrasound but said that it was too early to see the baby. I called this morning to get my bloodwork results and my level was 42 and he wants me to come back next Tuesday to do another ultrasound. He said the level needs to be about 1500 before they can see anything. Will it make it that high in only a week? Is something wrong he's not telling me? I'm nervous. We have been trying for about 9 months so we are so excited and don't want to lose this.