Scared and confused

Based on my LMP the doctor decided I’m somewhere around 4 weeks pregnant after receiving a positive pregnancy test. They took my blood today and my HCG was only at 18 which is usually a .5-1 weeks pregnant level amount. I took a test this morning and my tests keep getting darker, not lighter. Should I be worried this is chemical or the start of a miscarriage? Or could I simply just be earlier in the pregnancy than they thought. Can anyone explain? Thank you