7 weeks PP HCG

I’m 7 weeks PP and on birth control. We waited about a week, week and a half after starting BC to do anything because we were trying to be safe. This was our 4th baby. I started spotting last week off and on and assumed it was break through bleeding from BC but just felt funny so I tested yesterday and today and on every test there is the slightest faint second line… Does enough HCG stick around for this to be residual? My OB didn’t do an HCG test at my 6 week PP.