Very early and spotting

I had a faint positive last Tuesday and my blood work came back that hcg was 18.7. The doctor said I was likely only a week. I get another blood test today to be sure my levels are rising. Which I'm nearly positive they are because my test are getting darker and my digital said 2-3 weeks. Last night when I wiped I had very dark brown blood. I wore a panty liner and none has been on it. But I did have a little more when I wiped this morning. This is making me nervous. Has anyone had this happen and what was the outcome? I took another test this am and the line is still very dark.