Should I keep my appointment?

I found out I was pregnant last week.. hard dark positive and a positive digital test. I started bleeding Wednesday (no cramping), starting last night it’s just very light spotting. I made an appointment last Tuesday to get a scan (when I was still sure I was pregnant). I tested yesterday and the digital said not pregnant. I took a test (FRER) this morning and obviously it’s still positive.. if I’m not pregnant anymore and I was only 4 weeks how long will it take for the HCG to get out of my system?! This has never happened to me before, just wondering if I should keep my appointment or if I should just assume I’m not pregnant anymore.