HCG level

So I had a very faint positive test Tuesday and went for a blood test. The results came back that my HCG level was 18.7. So my doctor wants me to take another blood test Monday to confirm that my levels are rising. He thinks it was just really early. I'm still nervous that something may be wrong. My last cycle started August 8th so I should be farther along unless I ovulated late. Since my faint positive I have taken more urine test and they have gotten darker. I also took a digital yesterday and it said positive. So I'm thinking my levels are rising. Has anyone had something similar happen? Like low HCG levels at the start?