Many changes occur during sexual excitement in females like increase heartbeat, quick breathing, various muscle become tight, increased vaginal secretions, labia become flat and open, increase in the width and length of vagina etc . At this stage blood flow towards pelvis is increased and all these changes provide pleasure inside vagina. The orgasm begins with the muscle contractions. Rhythmic contractions also occur in vagina. You are feeling quivering in vagina due to contraction of the muscle during peeks of the excitement. Definitely, all this process is controlled by brain.