hcg in urine and blood...nor pregnant. chemical pregnancy?

In June I got vf lines on a Friday and Saturday with FRERs. Heavy bleeding started Monday and bHCG was 1. Now the same thing has happened to me again. Vvfl on wandfo and FRER last Friday. Ran in to get blood drawn before weekend. bHCG was 7. Told to come back for repeat on Monday. That one they said was 'undetectable' so I wasn't pregnant. Heavy bleeding started Tuesday. Both times this has happened, my luteal phase wound up 2-4 days longer than normal. I'm so confused now that this has happened twice. Were these chemical pregnancies? If not, where is the measurable bHCG in my blood and urine coming from, and why/how would that affect my luteal phase? Thoughts anyone?