Please read! Possible Miscarriage or Vanishing Twin...Drop in HCG

Christie
Long story short, found I was pregnant on Tuesday and had blood drawn. Came back at 152. This was actually really high for what I believed the conception date was which would have put me at less than 3 weeks from LMP. 
Did blood again on Friday and got results today. HCG dropped to 44 and progesterone is at 4.8.
Absolutely devastated.
 However, decided to take a Clearblur Digital (not the weeks one) because I remember reading their sinsitivty is a lot higher (50ml I think). Low and behold, it came out pregnant, two days after the blood work that came back at 44hcg.. 
Would it really take that long for HCG to drop to a level that wouldn't be read by that test by this point? 
Is it possible it could be a vanishing twin and maybe we still have a healthy pregnancy?
Any input or help would be much appreciated because I am distraught with this. We were so excited and now we don't know what to think.