Because you are pregnant.
The corpus luteum secretes the hcg to maintain progesterone levels till the placenta takes over production.
hcg is what pregnancy tests detect.
That depends why you're taking HCG. If it's for the weight loss and you're following the program:
If you're young it may be because of the diet, and it may need to be artificially induced later. That, or you might be entering menopause.