it actually due to increased blood circulation to the genitals
Estrogen
A dark bluish or purplish-red and congested appearance of the vagina and cervical muscosa. This is an indication of pregnancy. Marcy
Stress usually, or depending on the specific hormones you body produces
excess calcitonin or decreased parathormone
it occurs when the PARATHYROID HOROMONE (PTH) falls to low.