Miscarriage and HCG levels

Long story short, I miscarried March 9th. My husband and I had unprotected sex once on the 28th. Almost 3 weeks after miscarriage. I had my HCG levels checked the next day and they were still in the 400s. I go back this week to make sure they are 0. I just took a test today (4 weeks and 3 days after Mc) and there is still a very very faint line. Do you think my levels are just STILL not 0, or do you think it could be a new pregnancy?